M. K. A. Zittel 07i Fossil Lithistidce. 327 



Hyalotragos, Zitt. 



Tragos p. p., Goldf. (non Schweigger), Queust. et auct. 



Chenendopora p. p., Cupulospongia p. p., and Chenench'oseyithia p. p., 



From. 

 ? Cymbochlcenia, ? Bothrochlcenia, and Diacyparia, Poniv 



Sponge cup-, plate-, funnel-, or top-shaped, pointed or shortly- 

 stalked below. Upper surface depressed, with irregularly- 

 scattered larger and shallow, or with crowded smaller oscula. 

 Outer wall porous, or coated with a smooth, usually concen- 

 trically wrinkled covering-layer. In the middle of the de- 

 pressed upper surface the openings of a greater or less number 

 of vertical tubes which traverse the sponge-body to the base., 

 In the wall, parallel to the surface, very fine radial canals 

 run from the base to the upper margin ; and as these are fre- 

 quently arranged in radial vertical rows, a radiate structure 

 like that of Cnemidium^ but much finer and less distinct, is 

 produced. 



The skeletal elements, generally converted into calc-spar, 

 are rather large, curved, with several, pi'onged branches, but 

 with few spines on the shaft (see PI. VIII. fig. 6). They are 

 loosely interwoven, never grouped into fibroid trains, and pro- 

 duce a loose network well figured by Goldfuss (Petr. v. 10 6, 

 XXXV. 5 h) . Thus the whole sponge-body is traversed by a 

 capillary network of canals, and the skeleton really constitutes 

 only the very fine walls of these canals. Where the latter are 

 close together, as in the centre, they usually acquire a poly- 

 gonal form, and somewhat remind one of the tubes of Favosites. 

 It is only on the surface (and both on the outer and inner 

 surfaces) that the skeletal corpuscles are more closely inter- 

 woven, sometimes forming a smooth siliceous epidermis which 

 appears dense to the naked eye. 



This genus, which is very abundant in the Upper Jura, 

 differs from Gnemidiastrum chiefly in the absence of coarse 

 radial fissures ; and from the Cretaceous genus Verruculina^ 

 besides the different external form, by the looser, coarsely 

 meshed skeleton, the form and simple canal of the skeletal 

 elements and their grouping, and by the vertical canals. 



Goldfuss has described several forms under the Aristotelian 

 name Tragos^ previously applied by Schweigger to a living 

 horny sponge. Goldfuss, however, referred to Tracjos several 

 other siliceous and calcareous sponges ; so that the retention of 

 this name is inadmissible for two reasons. Quenstedt (Petref. 

 cxxviii. & cxxix.) figures the Upper-Jurassic species admi- 

 rably ; nevertheless, owing to their generally bad state of pre- 

 servation, their distinction is very difficult. The best speci- 

 mens arc from the Lower White Jura (zone of Ammonites 



