Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/112

68 latus:—Form conoidal or cylindro-conoidal, a thick outer wall surrounding a central hollow cavity. Wall more or less rounded off to an edge, sometimes produced at the circumference of the cormostome into a little rim, reminding one somewhat of the beard of

Holtenia in the same position. External surface: most exterior is a surface of fine texture, regularly marked with oblong rectangular depressions, which have well-defined edges, and are arranged not quincuncially but in a quaternary series; these cease on the upper surface of the wall, around the cormostome; the surface intervening between the oblong depressions and that surrounding the cormostome is pitted with innumerable minute punctations, corresponding to the hexaradiate elements. When the exterior surface is worn away, or when a section is made parallel to the sides of the wall, a system of irregularly winding cavities is exposed, which preserve more or less a circumferential direction; they arise from the suppression of a variable number of the vertical partitions between the oblong openings; internally they communicate with the great central cavity, or excurrent canal, by apertures which are well seen in a vertical section. When from the top of the wall its plain surface is removed, the canals beneath are revealed.

This species is very rare in the chalk, but in the Upper Greensand is by no means uncommon.

ii. V. (T. Smith).

No doubt can exist regarding the identity of our Cambridge Scyphia with this species; the simple and regular inner plaits of the chalk specimens are distinctly evident in our Greensand forms, as also are the large and hollow bosses raised on the outer surface at regular intervals.

iii. V. (T. Smith).

The Ventriculite arrangement of fibre is not so plainly exhibited in sections of this form as one could wish; but I think there is sufficient evidence to refer it to Ventriculites; and in its external characters, which are well preserved, it does not differ in the slightest detail from V. quincuncialis (T. Smith).

iv. V. (T. Smith); Porospongia ocellata (Seeley).

A section of P. ocellata reveals the Ventriculite structure in all its details; in some genus of the Ventriculidae it must consequently be placed; and from the thickness of its walls, the somewhat quincuncial arrangement of the oscula on its surface, and its general agreement with T. Smith's figures and description, I believe it to be a true Ventriculite, and different in no essential respect from V. cavatus (T. Smith). Its oscula vary considerably in size in different specimens; in one in my possession they are more than twice the diameter of those in another: this has been noticed as a peculiarity in the chalk forms of V. cavatus. The walls seem to be perforate.

Toulmin Smith proposes a classification of the chalk beds into upper, middle, and lower, based on the genera of Ventriculidae they