'68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, 



latus : — Form conoidal or cylindro-conoidal, a thick outer wall sur- 

 rounding a central hollow cavity. Wall more or less rounded off 

 to an edge, sometimes produced at the circumference of the cormo- 

 stome 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 open- 

 ings ; 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 Green- 

 sand is by no means uncommon. 



ii. V. MAMMiLLAEis (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. QTJiNCUNCiALis (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. cavattjs (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 quin- 

 cuncial arrangement of the oscula on its surface, and its general 

 agreement with T. Smith's figures and description, I believe it to b6 

 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 Ventriculidfe they 



