352 Mr. J. W. Davis on the Teleostean 



the pectoral fin from the shale above the West-Riding cannel 

 coal the fin-rays are very strong, and placed near together. 

 They present the characteristic chondroid structure ; and each 

 ray is composed of a number of separate segments, which do 

 not appear to have been joined together except by the in- 

 vesting cartilage of the fin ; the ends of the segments present 

 no articulating surface. The structure of the pectoral fins of 

 Pleuracanthus may be advantageously compared with the fin 

 of the mud-fish {Lepidosiren). 



The posterior or ventral fins are built up from the pelvic 

 girdle in a manner similar to the pectoral. There is a pair of 

 peculiar ventral shields, studded over with hook-like appen- 

 dages, whose use has not been clearly defined. In some of 

 the examples the ventral fins are much nearer together than 

 in others ; and where this is the case the bony appendages are 

 absent. It was suggested by Prof. Geinitz, in his great 

 work l Der Dyas,' that the ventral shield might have been a 

 sucker ; and he believed the genus to have been allied to 

 Cyclopterus. Dr. Kner, after examining all the specimens 

 then accessible, came to the decision that the ventral appen- 

 dages were hooking-organs, similar to the claspers found in 

 sharks and in the sheat-fishes or Siluroids of the present 

 day — this explanation being rendered probable from the 

 fact that some of the fossil fishes are provided with appen- 

 dages, whilst others are devoid of them, leading to the sup- 

 position that where the appendages were present the fishes 

 were males, whilst the females would be the fishes without 

 them. 



The vertebral column was continued from the pelvis to the 

 extremity of the caudal fin in a straight line. The vertebras 

 were cartilaginous and are not preserved. The rays sup- 

 porting the fin were to some extent osseous, and are distinctly 

 shown in some of the specimens. 



The systematic position of Pleuracanthus is one very diffi- 

 cult to define. The thick cartilaginous ventral appendages 

 incline to a resemblance to the Sharks ; but in the structure of 

 the skeleton, and especially in that of the fins, it does not in 

 any way resemble the Sharks. The osseous rays and interrays 

 which support the dorsal fin (the latter also being supported 

 by bony fin-rays), the bony ribs with expanded bases for 

 attachment to the vertebra?, and the possibility, indicated in 

 some specimens, that some of the vertebras themselves had 

 osseous centres, all remind us of a strong resemblance to the 

 type of bony fishes. The identification of the gill-arches 

 bearing teeth, the projection of the gill-rays, and the presence 



