agreement is the more remarkable from the fact that in seeking additional data for comparative study among the cephalopods of the Oomia beds, the indications of community furnished by the lamellibranchs are found to obtain no positive support. The cephalopods described by Waagen* from the Oomia Group, with the exception of the belemnites, do not comprise representatives of any of the genera obtained from the Uitenhage beds, and moreover, when brought into comparison with European forms, seemed to Waagen to indicate a Tithonian or Portlandian age. Stoliczkat first directed attention to the great similarity between a Trigonia collected by Wynne during the geological survey of Cutch, and the South African T. ventricosa (Krauss), and the identity of these was afterwards confirmed when the Oomia shell was definitely referred to T. ventricosa by Waagen,§ Feistmantel, and W. T. Blanford. Feistmantel, in 1876,** noted the close similarity between Trigonia herzogi from the Uitenhage beds and a Trigonia from the Oomia Group which I have recently described under the name T. mamillata. As already pointed out, T. mamillata is comparable with T. herzogi and the South American T. transitoria in the manner in which it exhibits characters somewhat intermediate between those of the Clavellatæ and the Quadratæ, but it bears a still closer resemblance to T. holubi sp. nov. Owing to their large and massive shells, and their well-characterised sculptural plan, the members of this group of Trigoniæ form a prominent feature in the faunas in which they occur, and supply significant data in the present comparative study. The basis for comparison is further strengthened by the association of true Costatæ with members of the Scabræ, both in the Oomia and the Uitenhage strata a fact to which I have already referred. T. parva accompanies T. ventricosa in Cutch, and although T. tenuis, another costate form, has not been recorded from any of the localities where T. ventricosa is found, it occurs in beds at least not older than those from which the remaining Oomia Trigonia have been obtained. In the Uitenhage Series we have T. tatei, which, although not closely comparable with T. tenuis, and distinguished by much coarser ribbing of the flanks, also shares some of the characteristics by which the Oomia form is contrasted with most of the * Waagen (1). Wynne (1), pp. 225, 231. || Feistmantel (2), p. 164; (3), p. xxxvii. + Stoliczka (2), p, 315 (1871). § Waagen (1), p. 237. ¶ Medlicott and Blanford (1), p. 261; (2), p. 224. See also Kitchin (1), p. 104. Feistmantel (1), p. 116. Costatæ from the Jurassic rocks of Europe. These distinguishing features are the elongated oval figure, the delicate sculpture of the marginal carina and area, and the elongated form and fine ornamentation of the escutcheon; they lend an aspect of similarity which cannot be overlooked, especially when these lingering examples of a typically Jurassic section are brought into comparison with the majority of European representatives. Other Trigonia which serve in most conspicuous manner to characterise the faunas under consideration, and certainly at first sight appear to furnish the strongest corroborative data in a correlation, are the members of the group of Trigonia v-scripta in the Oomia beds and the group of T. vau in the Uitenhage Formation. The similarity of one of the Oomia Trigonia to T. vau Sharpe seems to have been first pointed out by Feistmantel, and W. T. Blanford t shortly afterwards suggested that a shell from the Oomia Group, which was ascribed by Feistmantel to the genus Goniomya, might in reality represent the African T. vau. Which of three Oomia forms was here and elsewhere; referred to in this connection is uncertain, but it is possibly the one which I have described under the name T. v-scripta, § although the less well-known T. dubia in reality bears a much closer resemblance to T. vau, particularly in the outline. When dealing with the Oomia Trigonia I pointed out that T. v-scripta, when adult, more closely approaches an undescribed Uitenhage form which is associated with T. vau. This is T. stowi, described in these pages, and it is certainly closely allied to T. vau; the youthful characters of shape and ornament are practically identical in these two types, but a marked divergence is observed to accompany progressive growth, and the adult forms are perfectly distinct. The characters of the youthful period in T. v-scripta are also practically the same as in T. recurva, with which it is associated in the Oomia beds, and in this case likewise, wide divergence is exhibited as the adult characters are required. But the marked differences which exist between the essential features in the youthful stage of the two Uitenhage and the two Oomia shells respectively induced me, when describing the Cutch Trigonia, to distinguish between the "Group of T. vau" and the "Group of T. v-scripta." Subsequent examination of further material has strengthened the belief that we are here dealing with a case of convergent development, and that close alliance is by no means to be inferred from the *Feistmantel (1), p. 116. † W. T. Blanford (1), p. 118. Medlicott and Blanford (1), p. 261. § Kitchin (1), p. 70, pls. vii., figs. 6–8; viii., figs. 1-3. 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: Oomia Beds. 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 See also W. T. Blanford (1), p. 118. 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 Trigonie 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). * |