68 



polyps is ensured, together with a great increase of polypiferous 

 surface. 



I am aware of no instance among the Ventriculidse in which 

 a mere ridge or flap of membrane is added on, as this 

 would practically be if the membrane forming the sides of these 

 depressions came in contact with that forming the sides of the 

 plaits. There are, on the other hand, very many cases in which, 

 as in the present species, additional strength to the whole mass 

 is afforded by a means which gives a very great additional surface 

 for the development and security of that life, the manifestation 

 and multiplied means of enjoyment of which were the end of the 

 existence of the whole creature. 



On each side of the places where these meeting bulgings blend, 

 the vacant spaces between the plaits put on, naturally, from the 

 shortness and regularity of the distance between the bulgings, a 

 circular form. As one of these must, of course, lie between each 

 double pair of the depressions, a quincuncial figure is thus as- 

 sumed by the whole (see the darker spots in fig. 15 of PL XIII.). 

 The inquirer will thus at once understand the appearance pre- 

 sented by the inner surface of many species of Ventriculidse. He 

 will clearly see that the " tubuli " of Dr. Mantell are things with- 

 out existence. He will understand that though, at the surface of 

 a fossil of which all the interstices are filled with chalk, all the 

 circular marks appear alike, yet the dissection of alternate rows 

 of them will reveal very different conditions ; one row consisting 

 of more or less shallow (in the present species usually rather 

 shallow) and closed depressions; the other having no true de- 

 pressions at all, but consisting of cavities extending to the bottom 

 of the intermediate plait, and indeed, before and behind, under- 

 neath the intermediate bulgings, running into and forming part 

 of the longitudinal cavity of that plait. 



The wall of the variety tenuiplicatus being usually thin, the de- 

 pressions on the inner plaits, though shallow, are sometimes to 

 be seen through on the outside, if the specimen has come very 

 clean out of the chalk. It is very rarely, however, that specimens 

 can be got out of the chalk in this clean manner, portions of the 

 matrix usually filling up the spaces between the plaits. In the 

 recent state, and when, instead of being in the collapsed condi- 

 tion in which we find the fossils, the animals were alive and fully 

 distended with all- their fluids*, no doubt the lower surface of 

 all the depressions on the top of the inner plaits could be seen 

 from the outside. The living creature must have appeared as 

 composed of a number of plaits which, on their inner surfaces, 



* Every reader must perceive the difference between the comparative 

 states of coflapsion and distension and those of contraction and expansion. 



