330 M. K. A. Zittel on Fossil Lithistidoe. 



or basin-shaped. Both surfaces porous. Canal-system very 

 imperfectly developed, sometimes replaced by the loose texture 

 of the skeleton ; but frequently the water flows in numerous 

 serially arranged capillary tubes through the whole length of 

 the wall, which thus acquires a fibrous or radiate aspect (see 

 Goldf. xxxiii. 5 a). Skeletal elements like those of Hyalo- 

 tragos. 



This genus closely approaches both Hyalotragos and Cho- 

 nella. From the former it differs by its irregular but generally 

 lamellar form, and the absence of large oscula and vertical 

 canals ; from the latter, by the different structure of the skele- 

 ton. Thus in Pldtychonia the ratlier large moderately rami- 

 fied skeletal corpuscles are individually interwoven ; the much 

 smaller and more ramified and pronged elements of ChoneUa 

 readily group themselves into coarse fibres, giving the 

 skeleton a much denser structure. This structure, however, 

 produces a well-developed canal-system ; and in ChoneUa 

 perpendicular canals penetrate the wall from the pore-like 

 apertures of the surface. 



Various species of this genus occur in the Upper Jura ; but, 

 mainly owing to the state of preservation, their distinction is 

 a matter of much difficulty. Quenstedt formerly grouped them 

 together as Spongites vagnns] but recently he has distin- 

 guished several species under Planispongia (Petr. Bd. v. 

 pp. 317-323). 



1. Platychonia [Spongites] vagans, Quenst. Jura, lxxxii.8. 



2. Platychonia auriformis, Quenst. Petr. cxxxi. 1. 



3. Scyphia Schlotheimij Mltnst. Goldf. xxxiii. 5. 



4. Spongites triangulusj Quenst. Petr. cxxxi. 2. 



5. Spongites stragulus^ Quenst. ib. cxxxi. 9. 

 *6. Sjwngites feralisj Quenst. ib. cxxxi. 14. 



7. '^ Plococoelia obscura, Etal. Leth. Br. lix. 16. 



BOLIDIUM, Zitt. 



Amorpliospongia p. p., D'Orb., Rom. 

 Amorphofunffia p. p., From. 

 ?Lithosia, ? Cladolithosia, Pom. 

 Stellispouffia ^. ^. , Horn. 

 Sparsispongm, Gein. 

 Achilleum p. p., Reuss. 



Sponge nodose, with a rounded or warty surface, sometimes 

 ramose, thick. Surface with fine ])ores only. Skeleton of 

 small, nodular, curved corpuscles, branched at the ends. 

 Surface near the base often covered with a dense layer of 

 young interlaced skeletal corpuscles. 



