63 



(PL XIII.), the character represented in fig. 3 will, at another 

 place of section, be seen. 



6. Ventriculites cavatus. PL XIII. fig, 5. 



Memhrane folded in concavities of considerable breadth, rarely 



regular in disposition : wall of moderate thickness : moveable 



processes often well-seen. 



This species can never be confounded with the last. The shape 

 of the fold, its sloping edges, and the want, in general, of regular 

 figure, at once distinguish it. A linear arrangement of the de- 

 pressions is often traceable, marking the places of the primitive 

 plait. 



This species might readily be divided into two or three varie- 

 ties, marked by the fewness or abundance, the small or large 

 size, and the greater or less degree of regularity, of the folds. 

 The principle of the fold, which in all cases is the ground- 

 work of the present classification, appears however to be the 

 same in all these cases, and they may, not improbably, be but 

 modifications indicative of a difference of age. The concavities 

 are usually somewhat elongated in specimens of which the pouch 

 is closely cylindrical. 



In this case, as in all others, it is necessary that both sides, or 

 a good section, of the specimen should be examined, as V. radiatus 

 will be found to be marked on the inner surface by characters 

 bearing some superficial resemblance to those distinguishing both 

 surfaces of the present species, while V. bicomplicatus bears, both 

 on the external and internal surfaces, characters which may some- 

 times mislead the observer. A section of the present species how- 

 ever will show the wall much thinner and the cavities shallower 

 than in either of those species, while, in well-preserved specimens, 

 the entirely different character of the fold cannot fail to be seen. 



7. Ventriculites striatus. PL XIII. figs. 6 & 13. 

 Membrane folded from base to margin in regular close and sim- 

 ple plaits : processes very conspicuous : wall of moderate thick- 

 ness. 



This is an exceedingly interesting and very marked species. 

 The plaits, which have heretofore been traceable only by careful 

 comparison and an effort of induction, become in this species the 

 one characteristic feature. Not a trace is present, on either sur- 

 face, of the " tubuli" assumed to be polyp-cells or absorbents 

 of Dr. Mantell, and the presence of which is expressly stated by 

 him to be characteristic of the genus and species* ; upon the as- 

 sumption of which indeed all his views and descriptions have 



* See ante, p. 5. 



