208 



PORIFERA 



ends of the sixth rays, and at present problematic. An axial 

 canal is present in each of the rays — the six canals meeting at 

 the centre of the spicule. Special chinks between the spicules 

 appear to have provided a passage for the water current. 



The beautiful Ventriculites, so common in the Chalk and 

 present in the Cambridge Greensand, are historically interesting, 

 for the fact that they are fossil Hexactinellida of which the 

 general and skeletal characters were very minutely described by 



Toulmin Smith long before recent 

 representatives of the group were 

 known. In common with a num- 

 ber of fossil Dictyonine species 

 they are distinguished by the per- 

 foration of the nodes, a character 

 due to the fact that the siliceous 

 investment which unites the 

 spicules together stops short before 

 reaching the centre of each spicule, 

 and bridges across the rays so as 

 to form a skeleton octahedron. 

 This character is rare in recent 

 Fig. 103.— a node ot the skeleton of Hexactinellids, but, as first pointed 

 Ventriculites from the Cambridge o^^ ^y Carter, it is presented by 



Greensaud. (After SoUas.) ■' ' f. , . i y , 



one or two forms, of which Aulo- 

 cystis grayi Bwk is best known. The majority of the fossil 

 Hexactinellida belong to the Dictyonine section, a fact attribut- 

 able to the greater coherence of their skeleton. The " Dictyonina " 

 are to be reckoned among the rock -builders of Jurassic and 

 Cretaceous times. 



The Octactinellida and Heteractinellida are two classes 

 created by Hinde^ to contain certain little -known Devonian 

 and Carboniferous sponges, possessing in the one case 8 -rayed 

 spicules, of which 6 rays lie in one plane and 2 are perpendicular 

 to this plane ; in the othfer case, spicules with a number of rays 

 varying from 6 to 30. Bearing in mind the manner in which 

 octactine spicules are known to arise in recent Hexactinellida 

 (p. 200), it is clearly possible to derive these 8 -rayed spicules 

 from hexactines by some similar method ; while the typical 



"Monograph British Fossil Sponges," Patoeoai. Soc. xl. and xli. 1887 and 



1888. 



