ACANTHODII 189 



scales and the spines are probably of similar origin ; and ridge- 

 scales, somewhat intermediate in shape, may extend from the base 

 of the dorsal fin on to the head (Parexus). 



A varying number of spines may also be found between the 

 pectoral and the pelvic fin (Figs. 156, 164); their presence has 

 been taken to indicate the former continuity of these two fins (Cope, 

 A. S. Woodward [503]). It is to be noticed that there is no con- 

 centration of the base of the fins, no notching of the posterior edge 

 such as is found in other fish, with the exception of the Clado- 

 selachidae, the fin passing gradually into the body behind. 



The lateral line runs usually between two rows of scales, as in 

 Elasmobranchs (Fig. 156, C). But in Euthacanthus gracilis it was 



cr 



'--cf. 



FIG. 158. 



Restoration of Chciracanthus Murchisoni, Ag. (After Traquair.) u.f, anal, c./, caudal, 

 d.f, dorsal, pc.f, pectoral, and p.f, pelvic tin ; c.?', circumorbital ring ; y, five branchial slits. 



covered over and enclosed by double rows of modified scales 

 (Dean [111]). 



Of great interest is the skeleton, which shows certain unexpected 

 points of resemblance to that of the Osteichthyes. The notochord 

 appears to have been persistent and unconstricted ; there are 

 slender neural arches, and similar haemal arches thickened at the 

 base of the hypochordal fin. Traces have also been found 

 (Dean [111]) of dorsal radials forming *a row separate from the 

 vertebral column. Radials also appear in the front region of the 

 hypochordal fin strengthened with dermal scales. The skull and 

 visceral arches seem to have closely resembled those of the 

 amphistylic Elasmobranchs (Fig. 159). The palato-quadrate has 

 a large otic process, the hyomandibular is elongated, the five 

 branchial arches have the usual epi-, cerato-, hypo-, and basi- 

 branchial elements. Most remarkable is the strengthening of the 

 primary upper and lower jaws with hard bone-like tissue. This 

 tissue is probably never typical bone ; but it resembles it very closely,. 



