PRINCIPAL GROUPS OF THE SPONGES. 



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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 forming an elegant octahedron, which is 

 known as the " lantern " (see the same type of spicule in Ccelopty- 

 chium, fig. 59, b). The sponge -body in the Ventriailitidtz was 

 attached to the sea-bottom by a root of fasciculate flinty fibres, and 

 a dermal layer in the form of a cribriform siliceous membrane was 



Fig. 64. — J'entriczilites sj>. Whii 

 Chalk, Britain. 





ffliW 



Kill) 

 IK 



HiVt 



wfe? 



Fig. 65. — I'entriculites striatiis, 

 from the Cretaceous rocks of Han- 

 over, one-half of the natural size. 



present. Typical Cretaceous genera of the Ventriculitidce are Ven- 

 triculites itself (figs. 64 and 65), of which numerous species are known, 

 Rhizopoterion and Cephalites. 



Closely allied to the preceding is the family of the Caloptychidce, 

 comprising the beautiful Chalk Sponges which constitute the genus 

 Ccdoptychium (fig. 59). These are in most respects very similar to 

 Ve?itriculites, but have a flattened or discoid body supported on a 

 short stem, the skeleton being folded into laminated walls which 

 divide the central cavity into radial chambers. 



The group of the Mceandrospongidce. is also closely related to that 

 of the Ve?itriatlitidce, and comprises variably shaped sponges with 

 folded walls, the folds often anastomosing so as to give rise to open 

 tubes. The genera of this family — such as P/ocoscyphia, Tremabolites, 

 Etheridgia, Toulminia, and Camerospoiigia (fig. 66) — are mostly Cre- 

 taceous, though a few types occur in the Jurassic deposits. 



