484 THE DEPTHS OF THE SEA. [CHAP. x. 



such recent forms as Aphrocallistes, Iphiteon, Hol- 

 tenia, and Askonema with certain series of the chalk 

 Yentriculites, there cannot be the slightest doubt that 

 they belong to the same family in some cases to 

 very nearly allied genera. Pig. 80 represents a very 

 beautiful specimen of Ventriculites simplex preserved 

 in flint, for which I am indebted to Mr. Sanderson of 

 Edinburgh. Looking at this in the light of our 

 knowledge of Euplectella or ApJirocallistes beatrix, 

 we have no difficulty in working out its structure, 

 even to the most minute microscopic detail. 



Other sponges, belonging chiefly to the Lithistida3 



and the Corticatse, re 

 produce with wonderful 

 accuracy the more irre 

 gular sponge-forms of 

 the chalk and green- 

 sand ; and a group, as 

 yet undescribed, but 

 apparently an aberrant 

 family of the EsperiadaB, 

 send out long delicate 



FIG. 81. Ventriculites simplex, TOUI.MIN SMITH, tubes, Which Contract 



Outer surface ; four times the natural size. , . , 



slightly, but m a most 



characteristic way, at the point of their insertion 

 into the sponge body, recalling very forcibly the 

 peculiar manner in which the tube-like root processes 

 join the sponge in such genera as the vaguely defined 

 Choanites. 



One sponge belonging to the group is represented 

 at Pig. 83. A sphere 15 to 20 mm. in diameter con 

 sists of a smooth glossy external rind, composed of 

 closely meshed pin-headed spicules, with two kinds 



