J. W. DAVIS OH- THE GENUS PLETJRA.CANTHUS, AGASS. 325 



Tie considers may have served a similar purpose to that of the claspers 

 in cartilaginous fishes or the sheat-fishes. In some specimens they 

 are absent ; and these he concludes were female fishes. 



The zoological affinities of this genus have been the subject of 

 much careful research. Prof. Agassiz considered it as representing 

 ■a fossil Eay nearly related to Trygon. Dr. Goldfuss consigned 

 the genus to the Selachians, from its resemblance to Squatina : and, 

 more recently, Dr. Kner has contended that it constitutes a distinct 

 order between the Selachians and the Teleosteans, having many 

 features in common with each, and forming an intermediate link 

 hetween the cartilaginous and bony fishes. 



Pletjeacanthus, Agassiz (Davis). 



Sjnnes more or less circular in section, with an internal cavity, 

 •terminal at the basal extremity, extending towards the apex ; straight 

 or slightly curved, and gradually tapering to a point ; surface of the 

 spine, where not denticulated, smooth or finely striated. Implanted 

 portion of base short, with thinner walls than the exposed part of 

 the spine. Along some part of the surface there extend two rows of 

 denticles ; these may be widely separated and lateral, they may ex- 

 tend in close proximity along the posterior surface of the spine, or 

 the two rows may occupy any intermediate position between the two 

 -^specified. 



1. Pleuracanthus LJ2VISSIMTJS, Agass. Fig. 1. 



S2nne 8 or 9 inches long, -6 to *8 of an inch in largest 

 diameter at the base, tapering evenly to a fine point. Straight, oval 

 In section, with an internal cavity extending from the base to within 

 a short distance of the point. Anterior and posterior faces smooth 

 or finely striated ; their lateral junctions are armed with a row of 

 acuminate denticles (fig. 1), strongly curved towards the base, with an 



Fig. 1 — Pleuracanihus Icevissimus, Ag. 



Denticles of spine, enlarged. 



inclination towards the posterior face of the spine. Each denticle 

 occupies about -1 of an inch. Near the point they are smaller, and 

 at the opposite cxtremit}- somewhat larger. They extend from the 

 point along two thirds the length of the spine. 



M. Agassiz, who described this genus and species from an imper- 

 fect specimen obtained from the coal-shales near Dudley, lays con- 



