350 Mr. J. W. Davis on the Teleostean 



and that the Xenacantlius of Beyrich was evidently the same ; 

 and, further, that Orthacanthus, from the great difference in 

 the position of the denticles which extend along its dorsal 

 margin, might constitute the only additional genus to Pleura- 

 canthus. Since then a number of intermediate species have 

 been discovered, which conduce towards proving that there 

 can be no real division between Pleuracanthus and Ortha- 

 canthus, and that, consequently, the latter must be absorbed 

 in the former. 



The Bohemian specimens of Pleuracanthus appear to have 

 been about 18 inches in length. They had a broad flat head, 

 which contracted somewhat towards the body, the latter 

 being narrower laterally, but of greater thickness from the 

 dorsal to the ventral surfaces, and gradually tapering towards 

 the tail. There were two pectoral and two ventral fins; and 

 a dorsal fin extended along the back ; beginning immediately 

 behind the head, it encircled the tail and extended along the 

 ventral aspect, nearly or quite to the position of the ventral 

 fins. In front of the dorsal fin there was a straight fin-ray 

 or spine about 5 or 6 inches long in a fish of the size men- 

 tioned ; it was inserted in the neck, and appears to have 

 been supported by muscles or cartilage, there being no process 

 for articulation to a bony support. The spine in some in- 

 stances possesses a row of denticles along each lateral sur- 

 face ; and in others the two rows of denticles are placed on the 

 back of the spine and approximate closely to each other. It 

 is composed of very close-grained, dense, bony substance, 

 and, to all appearances, was not immediately connected with 

 the dorsal fin. Besides these two varieties of the spines, 

 which are found attached to the fossil fishes of the Continent, 

 and which were originally described by Agassiz, in the ' Pois- 

 sons Fossiles,' from detached spines found in the Coal-mea- 

 sures of Britain, others have been discovered in America, and 

 described by Dr. Newberry in the Geological Survey of Ohio, 

 also from the Coal-measures; and during the last year or two 

 I have been fortunate in finding, in the canned coal of West 

 Yorkshire, other specimens with the denticles in various inter- 

 mediate positions between the lateral (or Pleuracanthus type) 

 and the dorsal (or Orthacanthus type), which go far to pi-ove 

 that this difference in the position of the rows of denticles is 

 not of generic importance, and that the two must be united in 

 the genus Pleuracanthus. 1 have expressed these views more 

 fully in a paper recently read before the Geological Society of 

 London. 



The body of the fishes in some of the specimens was covered 



