8o 



PORIFERA. 



deposits is represented by very numerous forms. In this genus, 

 the sponge is simple or branched, there is no definite canal-system, 

 and the skeleton consists (where its structure can be determined at 

 all) of anastomosing calcareous fibres composed of comparatively large 

 three-rayed and four-rayed spicules surrounded by similar spicules 

 of small size. Little is known of the Pharetrones of the Carbon- 

 iferous and Permian deposits ; but the Upper Triassic beds have 

 yielded numerous forms belonging to such genera as Eudea, Ver- 

 ticillites, Peronella, Corynella, and Stellispongia. In the Jurassic 

 and Cretaceous rocks the remains of Pharetrones are often very 

 abundant, and belong to very numerous genera. Of the many 



Fig. 71. — Pharetrones. a, Trcmacystia D'Orbignyi, from the Upper Greensand, of the nat- 

 ural size ; b, Portion of the outer surface of the same, showing the pores and the spicules of the 

 dermal layer, enlarged thirty times ; c, Detached three-rayed spicule, belonging to the dermal 

 layer, enlarged seventy-two times ; d, Fragment of the skeleton-fibre of Eias7uostoma scitulum, 

 showing large three-rayed and four-rayed spicules with small filiform spicules, from the Upper 

 Chalk, enlarged fifty times. (After Hinde.) 



Jurassic genera, Corynella is perhaps the most important. In this 

 genus we have sponges allied to Peronella, but usually possessing 

 a distinct canal-system, and having the reticulated skeleton-fibre 

 composed of "minute, filiform, three-rayed spicules disposed gen- 

 erally parallel with each other in the direction of the fibre " (Hinde). 

 Other largely represented Jurassic genera are Stellispongia, Sestro- 

 stomella, and Lymnorea. Of the Cretaceous Pharetrones, the genera 

 Peronella and Corynella still hold a predominant place, but many 

 other types are present, such as Oculospongia, Tremacystia, Elasmo- 

 stoma, and Pharetrospongia. In the genus Tremacystia (fig. 71, a) 

 the sponge-body is formed of connected hollow segments, with a 

 common axial tube, and the wall is formed by a single layer of re- 

 ticulated calcareous fibres penetrated by numerous minute canals. 

 The dermal layer (fig. 71, b and c) is formed of relatively large 

 three-rayed and four-rayed spicules ; while the skeleton-fibres are 



