between the two families; and thus it shows the probability of the present classification of the Cyclostome Polyzoa being highly artificial. But until the distribution and life history of each described form has been determined, it is impossible to form a natural classification of the whole. Meanwhile, the present artificial classification must be used for lack of better.

FILISPARSA RAMOSA, d'Orbigny.
Plate I., fig. 1.

1853. Filisparsa ramosa, d'Orbigny, Bry. Crét., p. 819, pl. 756 figs. 18-22.

Zoarium of erect, flattened, solid branches, in one plane. Zoœcia only opening on one face of the zoarium. Apertures tend to arrangement in V-shaped bands across the zoarium, but their distribution is somewhat irregular; four or five in a series; distance between apertures of the same series one-half times to once the diameter of an aperture; between apertures of neighbouring series twice to four times. Zoccia immersed.

There are three fragments of this species, one of which is figured.

Distribution. Turonian and Senonian.

FILISPARSA FRAGILIS, Marsson.

Plate I., fig. 2.

1887. Filisparsa fragilis, Marsson, Bry. Rüg.; Pal. Abh., vol. iv., p. 35, pl. iii., fig. 8.

Zoarium of erect, cylindrical, or slightly flattened solid branches, in one plane. Zoccia only opening on one face of the zoarium. Apertures arranged in single series across the zoarium; four or five in a series; distance between apertures of the same series one-half times or less the diameter of an aperture; between those of neighbouring series twice-and-a-half to thrice. Zoccia immersed, but the boundaries between them are more or less apparent as ridges. One fragment out of seven is figured. Distribution.

Senonian.

GENUS DIASTOPORA, Lamouroux.

Diastopora is here used, as in the British Museum Catalogue, for Diastoporida with an erect uni-, bi-, or even multi-laminar zoarium, consisting of fronds or of very flattened branches.

DIASTOPORA COMPRESSA (Goldfuss).

Plate I., fig. 3.

1827. Ceriopora compressa, Goldfuss, Petref. Germ., p. 37, pl. xi., fig. 4.

1851. Ditaxia compressa, von Hagenow, Bry. Maastr. kr., p. 50, pl. iv., fig. 10.

Zoarium of erect bilaminar fronds. Zoccia immersed, and their boundaries not apparent, or very slightly so. Apertures irregularly arranged; the distance between the apertures in a proximal-distal direction is about once the diameter of an aperture; that in a direction at right angles to the last is once or less than once the diameter of an aperture. This definition does not distinguish D. compressa from Mesenteripora Hyselyi, de Loriol (Urg. Land.: Mém. Soc. helv. Sci. Nat., vol. xxiii., 1869, p. 40, pl. iii., fig. 1), from the Urgonian of Neuchatel; and from the figures it is difficult to see any difference between these two forms. But fig. 10 m. on pl. iv. of von Hagenow's work shows a piece of a zoarium with the apertures elongate in a proximal-distal direction. It is only described in the legend "Theil der Oberfläche am oberen Rande," but it looks as if this were a worn surface and the elongation of the apertures were due to the angle between the surface and the direction of the zoœcia at that level. It is possible that in M. Hyselyi the direction of the zoccia would be perpendicular to the surface for some distance, in which case a worn specimen would show circular apertures.

D'Orbigny's figure of Bidiastopora Campicheana (Bry. Crét., 1853-4, p. 800, pl. 784, figs. 6 and 8) also resembles Diastopora compressa and Mesenteripora Hyselyi. But Pergens, who has seen. d'Orbigny's specimens, identifies B. Campicheana with B. acuta, d'Orbigny, and re-figures the latter (Revision des Bry. Crét. fig. par d'Orbigny, 1890, pl. xi., fig. 7), showing it to be a form quite distinct from D. compressa and M. Hyselyi.

Gregory places as synonyms of Diastopora compressa the two forms figured by d'Orbigny as Mesenteripora compressa and M. neocomiensis (Bry. Crét., 1853-4, pl. 756, figs. 10-13 and 7-9); but both these forms have emergent zoœcia.

Among the material described the only specimen referable to this species is a small fragment of the base of a frond. The earliest zoœcia, as would be expected, differ from the later ones; they are further apart and slightly emergent. Near the edge of the fragment they are seen to be close together and immersed, with their boundaries slightly shown. The apertures, especially near the edge,

where they are slightly worn, are som: what elongated in a proximal-distal direction.

Distribution. Senonian.

IDMONEIDE.

GENUS IDMONEA, Lamouroux.

IDMONEA VIRGULA, d'Orbigny.
Plate I., fig. 4.

1851. Idmonea virgula, d'Orbigny, Bry. Crét., pl. 631, figs. 15-17. 1863. Reptotubigera simplex, de Loriol, Invert. Mt. Salève, p. 127 pl. xvi., fig. 3.

Zoarium of a single zoœcium-bearing ridge, without a selvage short (about thrice as long as broad), rapidly becoming broader distally. The only specimen of this species has a flat reverse face, but does not encrust any object. But its general shape and correspondence with d'Orbigny's figure of Idmonea virgula renders it probable that it was originally encrusting some perishable object which has since decayed. This raises the question whether other Idmoneida with flat or convex reverse faces, such as Crisina marginata, d'Orbigny, in some cases may not have been forms encrusting some object, such as seaweed, which has since perished.

Distribution. Neocomian and Cenomanian.

GENUS CRISINA, d'Orbigny.

The separation of Crisina from allied Idmoneid genera, especially from Retecava, is attended with much difficulty. The genus is here used as restricted by Gregory, and distinguished from Tervia and Reticrisina, which Gregory regards as subgenera. Tervia is here considered as equal to Crisina in systematic significance. Crisina thus regarded may be defined as a genus of Idmoneidæ, in which the zoarium is erect, and consists of solid branches, which are subcylindrical or moderately compressed; the reverse face is well developed, but does not overlie an axial rod" as in Retecava; there is no unpaired median nor multiple median series of apertures on the obverse face; the apertures are in single lateral series, and circular.

* See W. D. Lang, Geol. Mag., 1907, p. 125.

J. W. Gregory, Brit. Mus. Cat. Cret. Bry., 1899, vol. i., pp. 159, 168, and 176.

CRISINA CENOMANA, d'Orbigny.

Plate I., fig. 5.

1851. Crisina cenomana, d'Orbigny, Bry. Crét., p. 732, pl. 614, figs. 1-5.

1845. Idmonea disticha, Michelin, Icon. Zooph., p. 204, pl. lii.,

fig. 18.

1853. Idmonea calypso, d'Orbigny, Bry. Crét., p. 733, pl. 747, figs. 10-14.

1890. Idmonea cenomana, Pergens, Revision des Bry. Crét. fig. par d'Orbigny, p. 344, pl. xii., fig. 3.

Zoarium subcylindrical, and the reverse face is convex or flat; there is no keel on the obverse face; the zoœcia are immersed, and when the reverse face is flat the zoocial boundaries are clearly marked; the lateral series of apertures are quite regular, and just reach the middle line of the obverse face; there are about four apertures in each lateral series; the apertures are circular, all of one size; those of the same series are distant less than one diameter of an aperture; those of different series two or three diameters of an aperture.

This definition distinguishes C. cenomana from all cretaceous forms of Retecava as well as from others of Crisina. Those species of Retecava which most resemble it have a very slight median keel on the obverse face, except R. Abbotti (Gabb and Horn), (Journ. Acad. Nat. Sci. Philadelphia, ser. ii., vol. iv., 1860, p. 404, pl. lxix., figs. 45-47), but in this form there is a ridge down the obverse face of each zoœcium.

*

Two distinct forms are included under the above definition, and both occur in the material here described. In the first, resembling d'Orbigny's figure of C. cenomana, the zoœcial boundaries are hardly visible and the reverse face is convex. In the second (pl. xlv., fig. 4), resembling d'Orbigny's figure of Idmonea calypso, the zoocial boundaries are strongly marked and the reverse face is flat. The only justification for placing the two forms under one name is their identification by Pergens. Speaking of C. cenomana, under the synonyms of which he had placed I. calypso, Pergens says, "quelquefois les branches sont aplaties à la partie inférieure et forment ainsi une variété décrite comme I. calypso." That is, I. calypso is an earlier growth stage of C. cenomana confined to the proximal portions of the zoarium. In view of the very fragmentary character of the specimens usually obtained of these erect Idmoneids, * E. Pergens, Revision des Bry. Crét. fig. par d'Orbigny, 1890, p. 344,

it is exceedingly rare to get knowledge of the growth stages of any one form. But the specimens of C. cenomana here described do not look like proximal ends of zoaria. It is likely, then, that a form exists whose adult zoarium over its greater extent has the characters of I. calypso, d'Orbigny, while these are confined to the proximal parts of an adult zoarium of C. cenomana.

Distribution of both C. cenomana and I. calypso.

CRISINA EXCAVATA (d'Orbigny),

Plate I., fig. 6.

Cenomanian.

1853. Idmonea excavata, d'Orbigny, Bry. Crét., p. 742, pl. 749, figs. 11-15.

Zoarium subtriangular with a flat or concave reverse face; the lateral series are regular and contain six to eight apertures; the apertures are circular; the zoœcia are emergent. Idmonea filiformis, d'Orbigny (Bry. Crét., 1853, p. 744, pl. 750, figs. 1–5), and Idmonea lata, d'Orbigny (Bry. Crét., 1853, p. 734, pl. 748, figs. 6-10) resemble this form, but have only three apertures in a lateral series. Idmonea marginata, d'Orbigny (Bry. Crét., 1853, p. 744, pl. 749, figs. 20–23), with which Gregory unites this form, differs from it by having immersed zoœcia.

Distribution. Senonian.

CRISINA MARGINATA (d'Orbigny).
Plate I., fig. 7.

1853. Idmonea marginata, d'Orbigny, Bry. Crét., p. 744, pl. 749, figs. 20-23.

1853. Idmonea angulosa, d'Orbigny, Bry. Crét., p. 735, pl. 748, figs. 11-15.

Zoarium subtriangular with a flat or convex reverse face; the lateral series are regular and each contains four or five apertures; the apertures are circular. This form is very like C. excavata (d'Orbigny), but has fewer apertures in a lateral series, and the zoœcia are immersed.

Distribution. Turonian and Senonian.

GENUS TERVIA, Jullien.

Idmoneida whose zoarium is erect and consists of solid branches. There is an unpaired median series of apertures between the two rows of simple lateral series. Terria is considered by Gregory as a