174 



PORIFERA. 



On the other hand, in the Jurassic rocks the Hexactinellid Sponges, 

 though comparatively few in number in the lower division of the 

 system, attain a great development in the upper beds of the system, 

 in certain regions. Of the Jurassic Hexactinellids, Craticularia, 

 Verrucoaxlia, Ti-emadictyon (fig. 63), Sporadopyle, and Sphenaulax 

 possess a skeleton built upon the same type as the recent Eurete and 

 Fari-ea ; Pachyteichisma and Trochobohis are early forms of the great 



Fig. 63. — Portion of the skeleton of Tremadictyou reticulation, enlarged 50 diameters, from 

 the Jurassic rocks. (After Zittel.) The original spicules are soldered into a continuous trellis- 

 work by a coating of silica ; but their position and hexradiate form is shown by their axial 

 canals. The " crossing-nodes," or points of intersection of the arms of each spicule, are solid. 



family of the Ventricu/itidce ; Cypellia, Stauroderma, and others repre- 

 sent the extinct family of the Staurodermidce ; and Stauractinella 

 belongs to the group of Hexactinellids (Zyssah'na) in which the 

 skeleton-spicules are only united by sarcode, so that they do not 

 form a continuous network. 



In the Cretaceous deposits, and especially in the Chalk itself, the 

 Hexactinellids are very largely and abundantly represented. Of the 

 family of the EuretidcB, with their regular spicular mesh and simple 

 spicular nodes, we have now few forms (Crattcu/aria, Verrucoccelia, 

 &c.) ; but the great family of the Ventrkutitidce and the groups allied 

 to this now undergo a marvellous expansion. In the Ve?itriculitidce 

 proper, the sponge-body is of variable shape, but usually more or 

 less cup-shaped or infundibuliform (figs. 64 and 65), or cylindrical, 

 the wall being often folded. The skeleton-spicules are always united 

 into a continuous lattice-work, and their " crossing-nodes " are not 

 solid. On the other hand, the point of intersection of the arms of 

 each hexradiate spicule forms an open octahedron, in the centre of 



