The Primitive Fishes 



2«I 



dae is met with. The species of it may be said to be known 

 only by their teeth, except in one or two rare cases, where 

 parts of the head and the shagreen of the skin are pre- 

 served. From the usual absence of fin-spines beside the 

 teeth, they suggest possible derivation from a soft-finned 

 ancestry like the Cladodontidae. The richest in species 

 of the genera are — if identifications of teeth alone are to 

 be trusted — Janassa, Petalodiis, Ctenoptychhis and Poly- 

 rhizodiis. 



Janassa is a genus that extends from Lower Carboni- 

 ferous into Permian beds, and has been specially recorded 

 from the freshwater Permian deposit of Britain known as 

 the Marl Slate, also from the probably contemporaneous 

 Kupferschiefer of Germany and the " Boghead" of France. 

 Abundant "fern" and coniferous impressions, labyrintho- 

 dont imprints, and numerous fishes belonging to Janassa, 

 Platysomiis gihbosus (Fig. 45), Palaeoniscus sp. Acan- 

 thodes, Pleuracanthiis (Fig. 44) and Amblypterus, also 

 various as yet freshwater reptiles, lived side by side or de- 

 cayed together. Janassa bituminosa is a typical freshwater 

 fish of the Upper Permian, while decomposition and de- 

 structive chemical alteration of the oils from it and others 

 above noted, doubtless gave rise to the highly bituminous 

 rock in which they are often embedded. 



Fig. 45. Platysomus gibbosus, a freshwater chondrostean 

 ganoid from the Kupferschiefer and Marl Slate rocks of Permian 

 age in Central Europe. About one-fourth natural size. (Restored 

 by Traquair). 



