514 M. K. A. Zittel on Fossil HexactinelUda, 



Becksia, Schliit. 



Sitziingsber. d. niederrh. Ges. Bonn, 1868, p. 93. 



Sponge-body cup-shaped, at the base with spinous appen- 

 dages. Central space very wide. Upper part of the thin 

 wall even, towards the base with coarse rounded folds, between 

 which openings are left. These apertures are connected with 

 tubes which unite together in a horizontal hollow ring. The 

 wall of the cup in the tubes consists of regularly arranged 

 lantem-spicules, the arms of which are adorned with spines or 

 root-like processes. 



Becksia Scehelandi^ Schliit. Spongitariensch. des Miinster- 

 landes, p. 20, Taf. i. 5, 6, 7. 



Pleurope, Zitt. 



Sponge-body narrow, lamelliform, elongated, compressed, 

 with large, round or oval apertures upon the narrow lateral 

 surfaces. Base elongated, consisting of solid longitudinal 

 fibres with transverse unions. The wall of the upper part of 

 the sponge-body is formed of 3-5 regularly arranged layers of 

 coalescent sexradiates with octahedral crossing-nodes, leaving 

 between them large cubical meshes. This lattice-skeleton, 

 however, is coated on the outside w^ith more or less thick layers 

 of the root-tissue ; the latter is furnished at the surface with 

 numerous small ostia, and traversed by fine radial canals, 

 which, however, are not continued into the lattice-skele- 

 ton. The inside of the wall is naked, and provided with 

 numerous small ostia, which communicate with the meshes of 

 the latticed framework. The root possesses neither ostia nor 

 canals. 



Pleurostoma lacunosum, Rom. Kr. i. 12. 



DiPLODICTYON, Zitt. 



Sponge-body compressed, broad, with a thick, nodular 

 pedicle and flat base. The narrow sides, as in Pleurope, with 

 large rounded apertures. The wall of the compressed cup 

 consists of two different skeletal layers. The inner is formed 

 by regularly coalescent lantern-spicules with very thick smooth 

 ai'ms ; the outer one, on the contrary, is composed of irregu- 

 larly arranged sexradiates with solid crossing-nodes. These 

 sexradiates of the outer surface become more and more strongly 

 developed downwards, and form the material of the whole 

 root. The outer layer of the wall in the upper part of the 

 sponge-body is covered with numerous ostia of radial canals, 



