ACANTHODII. 8ELACHII. 39 



spersed in a close series of minute cusps, all apparently fused 

 with the membrane-calcifications in both jaws. The finest 

 specimens of all these genera have been obtained from the 

 Lower Old Red Sandstone. 



The Acanthodidae are almost destitute of the free spines 

 between the paired fins, rarely showing even rudiments. More- 

 over, in course of geological time the pelvic fins degenerate 

 exactly in proportion as the pectoral fins enlarge. In the 

 Lower Devonian representatives of Acanthodes (Mesacanthus), 

 for example, the pelvic fins are not much inferior in size to 

 the pectorals, and are placed nearly midway between the latter 

 and the anal. In the Lower Permian A. bronni the pelvic fins 

 become insignificant aod the pectoral fins enormous, while the 

 two pairs are even more closely approximated than in the 

 earlier forms. The Upper Carboniferous A. wardi (fig. 27) 

 presents an intermediate condition. 



OKDER 4. SELACHII. 



Palaeontology teaches that the skeleton of the modern 

 ^harks and skates is extremely specialized compared with 

 that of the early Elasmobranchs. Presumably, therefore, little 

 reliance can be placed on the anatomy of the soft parts in 

 seeking a clue to the primitive organization of the sub-class. 

 All these fishes are destitute of membrane-calcifications, even 

 when the teeth seem most in need of support. They are 

 characterized by the presence of a well-developed pelvis. 

 Their pectoral fins are di- or tri-basal, without a long seg- 

 mented axis; that is, the basal cartilages in direct contact 

 with the pectoral arch are normally three, sometimes two in 

 number.while the hinder (postaxial) border of the metapterygium 

 rarely exhibits even rudiments of fringing cartilages. In all 

 but the least specialized genera, the vertebral centra are well- 

 formed and strengthened by a secondary peripheral calcification ; 

 and secondarily-developed intercalary cartilages occur more or 

 less irregularly between the neural arches. The sharks and 

 skates thus represent a distinct order of Elasmobranchs, for 

 which the term SELACHII may be conveniently reserved. 



