similarity in the adult stage in members of these two groups, however peculiar and striking may be the characters which appear to unite them, and at the same time to differentiate them from all other sections of the genus with which we are acquainted. Since the members of these two groups appear to illustrate mere homœomorphy, their value as evidence in the correlation of the faunas becomes very much reduced, and if dissociated from the forms which accompany them, they could not well be considered to afford proof of contemporaneity. But whatever be the causes that determine the evolution along converging lines in shells which, by their youthful characters, betray a heterogenetic origin, we may in this case safely infer from their occurrence with an association of forms in so many respects similar, that they acquired their common characters at approximately the same time. Before concluding this comparison of the Uitenhage and Oomia Trigonia, attention may be directed to certain broad features of general habit which in some measure lend a distinctive aspect to several members of the genus in the faunas under discussion; and it will be noticed that while these features serve in great degree to imprint a facies on the assemblage which brings it into contrast with European occurrences, the same broad distinguishing characters are not confined to one section of the genus, but are shared by members of stocks not intimately related. There is the tendency to great posterior elongation of the shells, and in some cases a siphonal gape; the obliteration of the carina with disappearance of a definite demarcation between flank, area, and escutcheon; the dwindling and disappearance of sculpture on the area; and in several instances the situation of the umbones relatively far from the anterior extremity. In the Oomia beds these points are illustrated in varying degree in certain degenerate derivatives of Costatæ, and in the group of T. v-scripta; in the Uitenhage beds they are exemplified in T. vau, T. stowi, T. rogersi, and T. conocardiiformis. In both Trigonia vau and T. dubia the parallelism with the genus Goniomya cannot be overlooked, and I have previously suggested that if complete shells of these could be procured, they would be found to gape at the siphonal end; this idea is now supported by a specimen of T. stowi sent to me from the South African Museum, which is almost uninjured at the siphonal border, and which plainly exhibits a gaping habit. We do not find further aids to comparison amongst the few Oomia Mollusca which have already been described, and a detailed account of the remaining lamellibranchs collected by Wynne and Stoliczka from these beds is still in course of preparation; but I am indebted to the Director of the Geological Survey of India for permission to utilise the Indian specimens at present in my keeping for the purpose of this correlation. Several Oomia types at once suggest most strongly their affinity to Uitenhage forms, and these are the following. An Exogyra occurring in Oomia beds at the Trummo River is certainly not distinguishable from individuals of E. imbricata Krauss, and may at any rate be thus provisionally named; an Astarte found in association with Trigonia ventricosa and other characteristic Oomia forms very closely resembles Astarte herzogi Krauss, though it is not identical with it; there are fragments of a large, coarsely-ribbed Cucullaa which, so far as can be seen, shares all the distinguishing features of the strongly characterised Cucullæa kraussi Tate; a large Gervillia very closely resembles G. dentata Krauss, and is probably identical with it; and lastly, the Oomia beds have yielded specimens referable to the genus Seebachia, otherwise only known by Seebachia bronni (Krauss), from the Uitenhage Series. Two of these individuals from Cutch very closely resemble S. bronni, and it is not improbable that they are identical with it. The closely similar character of these lamellibranch-faunas is clearly shown when we place side by side the identical, proximate, and analogous types, as follows: Uitenhage Beds. Exogyra imbricata Cucullaa kraussi Gervillia dentata Astarte herzogi * Trigonia ventricosa Trigonia holubi Trigonia herzogi Trigonia of the vau group Seebachia bronni Oomia Beds. Exogyra imbricata ·Cucullaa kraussi Gervillia dentata? Astarte sp., near herzogi }(Pseudo-quadratæ) Trigonia mamillata Trigonia of the v-scripta A more critical and detailed study of the Oomia lamellibranchs may possibly reveal further connecting links, but two inferences may already safely be drawn from the general agreement observed to exist between these geographically widely separated faunas. Firstly, despite the absence of clues to correlation derivable from the Cephalopoda, we may conclude that the faunas were approximately con temporaneous; and secondly, that the lines of intercourse between the two areas were probably much more direct than was formerly believed to be the case. This correlation appears therefore to be in conflict with Waagen's conclusions concerning the age of the marine Oomia beds, which were thought to be Portlandian, but a brief consideration will show that this difficulty is more apparent than real. Doubts might perhaps be expressed as to the validity of a comparison based solely upon lamellibranch types, and it must be admitted that in many similar cases it would be right to place greater confidence in the evidence of cephalopods; but in the present instance the lamellibranchs compared are peculiarly well characterised, and indeed comprise no single form known in the European area. On the soundness of the evidence which these well-marked types afford, and upon which the above conclusions are founded, I think no doubt can reasonably be cast. The Mollusca of the marine Oomia beds, shown by Stoliczka to occur in the lower part of the group, cannot be said to give such definite indications of geological age as are to be derived from a study of the Uitenhage fauna. The ammonites from these lower Oomia beds were considered by Waagen to show close affinity with Upper Jurassic forms in Europe, and he believed some of the Trigonia to corroborate fully his view concerning the Portlandian age. But a detailed study of the Trigonia has shown that this belief was not well founded, and the relationship of any of the Oomia forms to the Portlandian Gibbosæ is at the best a matter for conjecture. I have provisionally referred one of these Trigonia to the group Gibbosæ on the strength of a certain broad similarity of characters, while at the same time recognising the possibility that this form may represent an aberrant derivative of some costate stock. Judged by analogy with Trigonia retrorsa, which can only be regarded as a degenerate costate type, the relationship of T. spissicostata to the Gibbosæ is extremely doubtful, but nothing more definite on this point can be said until material can be collected in a sufficiently favourable state of preservation to throw light on the nature of the youthful stage. Other Oomia Trigonia which exhibit characters simulating those of the Portlandian Gibbosæ have been shown to be allied to the section Costatæ, and nothing quite comparable with these diversely modified derivatives is known in Jurassic rocks, though the late adult stage of T. peninsularis Coq., from the Aptian of Spain, shows an analogous obliteration of sectional features which was regarded by Lycett to indicate degeneracy. The Waagen (1), p. 233 (1875). * value of Trigonia mamillata as an indication for age has already been considered, and the supposition of the Cretaceous rather than the Jurassic affinities of this form receives emphatic support from the presence of Trigonia ventricosa and T. pulchra, both members of the section Scabræ. Though claiming corroboration of his views from Trigonia which he supposed to be related to Portlandian forms, Waagen did not state that these were found in actual association with his Oomia ammonites; and the fact that none of the critical ammonite-species upon which he relied in his correlation is recorded from any of the localities-such as Goonaree, Oomia, or Huroora,where the Trigonia-beds are well developed, gives room for the suggestion that the ammonites and lamellibranchs may not represent horizons of quite the same geological age. Further, if we examine Waagen's descriptions and figures of the four ammonites which were thought to represent European forms, it may perhaps be allowed. that too much reliance has been placed on the evidence they were supposed to afford. Two were referred only with doubt to the respective European species; of the remainder, one represents a type of Perisphinctes which, so far as can be judged from the description and figure, does not justify the definite conclusions drawn by Waagen concerning its relation to a European Portlandian form. The other, a single specimen referred to the Tithonian Perisphinctes eudichotomus Zittel, is so preserved that the lobe-line is not visible, and a comparison of Waagen's figure with Zittel's original specimen in the Paleontological Collection of the State at Munich shows that the Indian form is rather thicker and more involute, though the agreement is otherwise good. It is scarcely necessary to dwell at any length on the question of the alleged discrepancy between the evidence of the plant and animal remains in settling the age of the Oomia beds, since this matter has already been so fully dealt with. The plants of the Oomia group were obtained from strata for the most part above the marine beds which yielded the cephalopods and lamellibranchs, but they led Feistmantel to refer these beds to the Middle Jurassic. Even should a revision of the Oomia plants prove the correctness of Feistmantel's view that the flora exhibits Oolitic affinities, there are many reasons why the evidence of the marine fauna must be allowed to outweigh that of the plants in a correlation with European stratigraphical standards.* Putting on one side the ammonites, the exact bearings of which on this question are somewhat doubtful, it may be said that there is * W. T. Blanford (2). nothing in the fauna of the Oomia Trigonia-beds to contradict the results reached by a study of the Uitenhage Mollusca, and a consideration of all the available evidence seems to justify the conclusion that here also we are dealing with the part equivalent of the Neocomian in Europe. (c) Traces of a Related Fauna in the Godavari District and in Hazara.-Traces of the Uitenhage-Oomia lamellibranch-fauna, as principally indicated by the presence of Trigonia ventricosa, have also been found to occur in an outlier of the Tripetty beds about 24 miles north-east of Coconada near the south-east coast of the peninsula of India.* Trigonia ventricosa is said to be here accompanied by Trigonia smeei, a characteristic fossil of the Oomia Group in Cutch, though it seems possible that this may be T. crassa, an Oomia form of similar type. The small collection of fossils from near Coconada was examined and named by Stoliczka, and it comprises Inoceramus and a few other lamellibranchs as well as Helicoceras and other remains of cephalopods. Trigonia ventricosa has also been recorded from strata exposed in the Margalla Pass in Hazara (N.-W. Himalayas), where it is said to be found in profusion. The bearings of these occurrences in a discussion of the broad question of distribution will be considered below. (d) Neocomian in German East Africa.-The Lower Cretaceous fossils collected during W. Bornhardt's journeys in German East Africa (1895-1897), and described by G. Müller,§ are of special interest in the present connection, because they exhibit in some measure a Uitenhage facies and also furnish strong links with the molluscan fauna of the Oomia Group. The remains of Cephalopoda are unfortunately very scanty and again fail us as a basis of comparison, but important links are found in some of the lamellibranchs, which include well-characterised Trigonia. Foremost amongst these is Trigonia ventricosa, occurring at a locality 8 km. north of the Nkundi stream, 29 km. north-west of Kiswere, in strata ascribed to the Lower Neocomian and brought into correlation with the Uitenhage beds by Dr. Müller. Trigonia beyschlagi G. Müller, which here accompanies T. ventricosa in great abundance, appears to have no counterpart in the Uitenhage beds, but it clearly belongs to the same category of modified Costatæ as T. smeei || and T. crassa¶ from the * King (1), p. 229; Feistmantel (2), p. 164; Feistmantel (3), p. xxxvii. Wynne (2), p. 125; Medlicott and Blanford (1), p. 503. § G. Müller (1). J. de C. Sowerby (3), pl. lxi., fig. 5. Kitchin (1), p. 44; pl. iv., figs. 4-6, pl. v., figs. 1-3. |