GREGORY, PRESEXT STATUS OF ORIGIX OF TETRAPODA 343 



ELASMOBRANCHII 



In the earliest known elasmobranchs, the Acanthodii of the Upper 

 Silurian and Devonian^ the locomotive apparatus as a wliole was in a 

 much more advanced stage than in the typical Ostracoderrai or Arthro- 

 dira. The locomotive part of the body, namely, the trunk and tail, is 

 from three to six times as long as the cephalo-pharyngeal region and no 

 heavy thoracic armor impedes the imdulation of the fusiform body. In 

 the earliest forms the median and paired fins are of large size and 

 unusually numerous, for in addition to the two dorsal fins, the anal and 

 the caudal fin, there is a whole row of accessory paired fins, or fin spines, 

 ])etween the pectorals or pelvics, suggesting the former presence in this 

 region of paired ventral fin-folds. The Acanthodii are, however, defi- 

 nitely excluded from ancestry to the higher types, by the tact that the 

 exoskeleton was more highly developed than the endoskeleton. The an- 

 terior borders of all the fins both median and paired were supported by 

 lln-spines, which are believed (Dean, 190T, p. 216) to represent clusters 

 of originally metameric dermal tubercles. Functional pectoral and pelvic 

 girdles were also developed from dermal elements, and perhaps served for 

 the insertion of powerful muscles, as well as for the support of the heavy 

 spines. But the underlying cartilaginous elements were little if any 

 developed and in my view the dermal elements of the girdles were 

 analogous but not homologous with those of Osteichthyes. In some 

 acanthodians {Gyracantlius) the pectoral fin-spines became overspecial- 

 ized and attained a relative!}' enormous size ; in other lines all the spines 

 were reduced and the body in the later types became much elongate as in 

 many other decadent groups of fishes (A. S. Woodward). 



In brief the Acanthodii failed to carry the exoskeleton beyond a low 

 stage of evolution, and their dermal shoulder-girdle was developed inde- 

 pendently of that of the Osteichthyes. 



The Cladoselachii avoided the line of specialization typified by the 

 Acanthodii and indeed went to the other extreme in sacrificing a large 

 part of the exoskeleton. But they carried much further a process which 

 in the Acanthodii was barely begun and soon abandoned, namely, the 

 building up and calcification of rods of cartilage, lying between the 

 myomeres and extending out into the median and paired fins ; these 

 metameric cartilaginous rods reached almost to the tips of the fins, thev 

 were jointed at tlie l3ody line and within tbe trunk they undei'went more 

 or less coalescence and enlargement, giving rise to the cartilaginous 

 girdles, basals and radials. In this group the paired fins, like the median 

 fins, had a wide base which was not exserted posteriorly from the bodv ; 



