Affinities of the Genus Pleuracanthus. 355 



in the Rays, through many intermediate stages, to those having 

 two rows placed very near together along the posterior sur- 

 face ; in others there is only a single row of denticles along 

 the posterior surface; and, again, there are many examples 

 without any denticles. These variations offer a curious 

 parallelism to the fossil examples ; and if, as I have else- 

 where hinted, the genus Compsacanthus of Newberry, with 

 only one row of denticles, should turn out to be a closely 

 related genus to Pleuracanthus, it will be still more so. I 

 have also recently acquired some spines with the same dense 

 structure as the Pleuracanthus , and similar inform, but devoid 

 of any denticles so far as I have ascertained, though I have 

 not been able to get one quite clear of the matrix. 



In the peculiar group of fishes Ischyodus the spine in some 

 respects very much resembles the Pleuracanths ; it is situated 

 immediately behind the head, is nearly straight, and has two 

 rows of denticles situated laterally on its upper part. A large 

 proportion of its length is without a fin-groove. Nearer its 

 base, however, there is a cavity for the fin which extended 

 behind the spine. Its base is expanded into a rounded articu- 

 lating surface, and fits into a socket composed of a strong 

 cartilage springing from the vertebra? — an arrangement which 

 allows of very considerable motion, and enables the fish to 

 elevate and depress its spine at will. The joint is similar in 

 the main to that of the Siluroids, but is less complex and de- 

 void of the interlocking apparatus characteristic of that group. 

 The Ischyodus occurs in the Lias and Chalk formations ; and 

 though the Chimeroids offer few resemblances to either the 

 Pleuracanths or Siluroids, its spine appears to occupy, in 

 some respects, an intermediate place between the two. 



As already observed, the position of the eyes in Pleuracan- 

 thus has not been identified. This may be due in great mea- 

 sure to the parts surrounding the orbit being of a soft carti- 

 laginous nature, and filling up the orifice previous to its fossi- 

 lization ; but it may also be well to remember that the eye 

 in the Siluroid fish is very small compared with that of the 

 Sharks or Rays, and for this reason it would be more likely 

 to become obliterated. 



There are, then, the following conspicuous points of rela- 

 tionship with the Siluroids : — the long tapering figure of the 

 fish ; its broad depressed head with rounded snout and termi- 

 nally situated wide mouth ; the naked skin, or more rarely 

 covered with minute enamelled, rhomboidal, detached plates ; 

 the absence of scales ; the peculiarities of the spine ; the long 

 unpaired fin extending along the back, encircling the straight 



