338 



ORDERS OF FISHES. 



been referred to another genus under the name of Thelodns 

 (fig. 295, B). In the same bed are found jaw-like bodies, with 

 tooth-like processes of different sizes, which have been named 

 Plectrodus, and have been supposed to be the jaws of fishes. 

 The true nature of these, however, is doubtful, and they cer- 

 tainly do not belong to Plagiostomous fishes. Possibly they 

 are the prickly margins of the cephalic bucklers of Cephalas- 

 pidean fishes. 



It should be mentioned, also, that M. Barrande enumerates 

 Ctenacanthus amongst the fishes found in the Upper Silurian 

 rocks of Bohemia, this genus being otherwise only known in 

 the Devonian and Carboniferous formations. 



In the Devonian Rocks the remains of Cestraphori are not 

 uncommon. The more important fossil spines of the deposits 

 of this period have been referred to the genera O/ic/ius, Ctena- 

 canthus, Homacanthus, and Cosmacanthus. The fossil teeth 

 belong chiefly to the genera Ctenodus, Ctenoptychius, Cladodus, 

 and Conchodus. 



In the Carboniferous period the remains of Cestraphori are 

 comparatively very abundant, though confined generally to 



"^^^ 



Fig. 296. i. Fin-spine of Pleiiracanthus (one of the Rays); a. Gyracanthus; 3. 

 Cttnacanthus ; 4. Tooth of Petalodus; 5. Psammodtis; 6. Cttnoftyckius. All from the 



Carboniferous Rocks. 



particular localities. The spines of the Carboniferous strata 

 have been referred to many genera, of which the most import- 

 ant are Cteiiacanthus (fig. 296, 3), Gyracanthus (fig. 296, 2), Ho- 



