146 



PROTOZOA. 



Eurete and Farrea ; Pacliyteicliisma and Trocliobolus are early 

 forms of the great family of the Ventriculitidce ; Cypellia, 

 Stauroderma, and others represent the extinct family of the 

 Staurodermidce ; and Stauractinella belongs to the group of 

 Hexactinellids 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 Euretidce we have now 

 few forms (Craticidaria, Verrucoccelia, &c.) ; but the great 

 family of the Ventriciditidce (employing this term here in a 

 general sense for the groups allied to the Ventriculitidw 

 proper) undergoes a marvellous expansion. The shape of 

 the sponge - body in this family is very variable, but is 



usually more or less cup-shaped, 

 infundibuliform, or cylindrical, the 

 wall being often folded (fig. 41). 

 The spicules of the skeleton are 

 always united into a continuous 

 lattice- work, and their " crossing- 

 nodes " are not solid. On the 

 other hand, the point of inter- 

 section of the arms of each hex- 

 radiate spicule forms an open 

 octahedron, in the centre of which 

 the central canals of the six rays 

 form a delicate axial cross. The 

 boundaries of the central space 

 are formed by twelve oblique 

 uniting beams, the whole forrn- 

 ing an elegant octahedron, which 

 is known as the " lantern " (see 

 fig. 38, B). The most import- 

 ant of the Cretaceous genera 



of the Ventriculitidce using the term in the above wide 

 sense are Ventriciditcs (fig. 41), Ccphalites, Cceloptychium, 

 Ccdlodictyon, Marshallia, Pleurope, Plocoscyphia, Etheridgia, 

 Oamerospongia (fig. 42), and Tremabolites. Very few Tertiary 

 representatives of the Ventriculitidce have hitherto been 



Fig. 41.- Ventriculites simplex. 

 White Chalk, Britain. 



