182 



PLE URA CANTHODII 



is modified and prolonged, much as in the Selachians, to form a 



clasper armed with sharp hooks. 



Ceratotrichia are present in all the fins of Xenacanthus, but 



have not been described in Pleura- 

 canthus. There is evidence from 

 the fossilised faeces of the pres- 

 ence of a spiral valve. 



Cladodus (Traquair [464]), 

 which seems to be allied to Pleura- 

 canthus, and has a similar dentition 

 and head -skeleton, differs con- 

 siderably in the structure of the 

 pectoral fin. The pectoral girdle 

 is of the typical form (Fig. 87). 

 The fin -skeleton consists of a 

 number of anterior radials, whose 

 proximal joints are incompletely 

 fused to a basal articulating with 

 the girdle ; behind it articulates a 

 second basal formed by the coales- 

 FIG. 147. cence of more posterior radials. 



Skeleton of anal fins of I'lcuraranthn* It is the base of a long many- 

 (Xenacanthtis) Declicni, Goldf. a, 7>, c, haemal . , j , > ,-, 



arches ; 1-6, segments of (in-radials. Right JOinted axis, which represents the 



(After me tapterygium and may have 

 been lodged in the body -wall. 

 This axis has presumably arisen from the bases of a single 

 series of radials, which have almost disappeared behind ; in this 

 case the fin is strictly uniserial. On the other hand, it may have 

 been derived from a mesorachic fin, like that of Pleuracanthns, 

 by the suppression of the postaxial radials. This on the whole 

 seems to be the more probable explanation of its structure. 



The pectoral-fin skeleton of Sijmmorium, which is supposed to 

 be allied to Cladodus, has been described by Cope [92] ; but it is 

 much more like that of a Selachian (Fig. 87). 



That the Pleuracanthodii are closely related to the Selachii is 

 shown by the structure of the skull and visceral arches, and the 

 presence of a pelvic clasper ; but the unconcentrated character of 

 the radials of the median fins, and the persistence of the girdles in 

 two halves, point to their having been derived from a common 

 ancestral form more primitive than any known member of that 

 Order. In these two respects they approach the Holocephali. The 

 ancestral Chondrichthyan, parent both of the Elasmobranchii and 

 of the Pleuracanthodii, must have had an amphistylic skull, and 

 paired fins with concentrated radials forming a fairly well-defined 

 axis. Whether a postaxial series of radials was present in the 



