GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 345 



approaches the hypothetical generalized form which Schmaihansen (1913, 

 p. 66), after thorough studies of the musculature and skeleton of the 

 caudal fin of fishes, regards as antecedent to the heterocercal tail of 

 t3'pical elasmobranchs. 



That the pleuracanths are truly sharks, although of a peculiar order, 

 is definitely proved by their skull-structure, which has been cleared up 

 by Hussakof (1911). As for their pectoral fins, the general resemblance 

 to those of Ceratodus offers an excellent example of convergent evolu- 

 tion : the protrusion of the metapterygial axis, perhaps followed by a 

 process of asymmetrical budding, having resulted in both phyla in a 

 more or less mesorhachic or biserial fin ; so that we may regard this type 

 as a "morphon" which has arisen independently in widely removed phyla. 

 The pelvic fins of PI eur acanthus, and indeed of all sharks, are, according 

 to the view here adopted, in a lower stage of evolution, since their bases 

 are less widely protruded from the body and they have departed less 

 from the fin-fold type. 



The large and high coracoscapular cartilage of pleuracanths remains 

 separate from its fellow of the opposite side ; in form it parallels that of 

 Tetrapoda and supports the large pectoral limbs, which have a single 

 proximal basal piece analogous to the humerus. If the pleuracanths had 

 happened to develop dermal plates around the shoulder-girdle we should 

 have had still more resemblances to Ceratodus and the Tetrapoda, to lead 

 further astray investigators who neglect the '^potency of convergence." 



In brief the elasmobranchs show a marked advance over the ostraco- 

 derms in all locomotive adaptations. In the development of median and 

 paired fins the earliest elasmobranchs overemphasized the exoskeleton; 

 some of the later ones on the contrary neglected the exoskeleton (Clado- 

 selachii, pleuracanths), but developed the endoskeleton to a high stage. 

 The basal cartilages, which were formed between the myomeres of the 

 fins, coalesce, enlarge and are widely protruded from the body to form 

 pectoral paddles either of tribasal, or rarely, of sub-mesorhachic type. 

 The pelvics remain on a lower stage of evolution, retaining usually a 

 broad base and acquiring an over-extended metaptervgium. 



ACTINOPTERYGII 



The earliest Actinopterygii preserved that fortunate balance between 

 endoskeletal and exoskeletal structures which, as Dr. Smith Woodward 

 (1906) has shown, was essential for the highest development. First they 

 either inherited or reinvented bone-cells, which, being carried to almost 

 any desirable point by the mesenchjrme and vascular system, served to 

 reinforce the exoskeleton and to replace the cartilaginous endoskeleton. 



