﻿DIPLACANTHID^. 23 



B. Paired spines between pectoral and pelvic fins 

 well developed. 

 Anterior dorsal fin-spine not exceeding the 



posterior in length Climatius (p. 28). 



Anterior dorsal fin-spine much exceeding the 



jrior in length Parexus (p. 33). 



Genus DIPLACANTHUS, Agassiz. 

 [Poiss. Foss. V. G. E. 1844, pp. 34, 40.] 

 Syn. Rhadinacanthus, R. H. Traqnair, Geol. Mag. [3] vol. v. 1888, p. 512. 



Body fusiform, probably not much laterally compressed. Teeth 

 minute or absent ; orbit with ring of four circumorbital dermal 

 plates. Pectoral fins large, and a median pair of stout spines fixed 

 between these to the basal pterygia ; a pair of free spines situated 

 ventrally between the pectoral and pelvic fins. 



The pectoral fins in this genus are somewhat difficult of interpre- 

 tation, but the accompanying woodcut (fig. 3) seems to represent 



Fig. 3. 



Pectoral arch and fins of Diplacanthus striatus, Ag.—b, basal cartilage ; 

 cl. clavicle ; i.cl, infraclavicle ; m, median spine ; s, fin-spine. 



the arrangement of the spines and pectoral arch in the type species. 

 As is usually the case in crushed specimens, the fin-spines are exhi- 

 bited from the dorsal aspect, and the ascending limb of the pectoral 

 arch is bent forwards and exposed from the inner side. The greater 

 portion of the pectoral arch consists of a pair of vertically elongated 

 elements (cl), each having a straight rod-like axis, filled with calcite 

 in the fossil, and thus originally either hollow or occupied by uncal- 

 cified tissue ; behind this axis there is a thin laminar expansion 

 of bone, diminishing upwards, and apparently extending downwards 

 and inwards to form an inferior limb. The pair of large bones does 

 not meet in the median line below, but is separated by a much 

 smaller pair of bony laminae (i.cl.), united in a finely dentated 



