120 



VERTEBRATE ZOOLOGY 



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these primitive sharks is the paired fin system 

 of Cladoselache, a fish that on the whole is 

 generally admitted to be the most primitive 

 of all true fishes, though its remains were 

 found in the Devonian rocks milUons of years 

 later than the earUest ostracoderms. The 

 paired fins of Cladoselache (Fig. 65) are so- 

 called "lappet-fins," broad based and closely 

 resembling in bony framework the median 

 fins, such as the dorsals. The skeleton of these 

 fins consists of two sets of elements. The 

 slender, nearly parallel, unjointed cartilages 

 occupy a distal position, and proximal to these 

 is a less numerous set of shorter and stouter 

 cartilages imbedded in the body wall and cor- 

 responding to the large cartilages (propte- 

 yrgium, mesopterygium, and metapterygium) 

 of the modern shark. The pectoral fin of 

 Cladoselache is not quite so primitive as the 

 pelvic and furnishes a transition between the 

 latter and the fins of modern elasmobranchs. 

 The location and general arrangement of the 

 paired fins of Cladoselache have given rise to 

 the theory that "the fins of fishes arise from 

 lateral skin folds of the body, into which are 

 extended internal stiffening rods." These 

 folds are supposed to be essentially like those 

 composing the median fin system and are be- 

 lieved to have been at one time continuous 

 with them, as in the hypothetical case de- 

 scribed by Dean. This continuous fold is sup- 

 posed to have been specialized in two regions 

 to form the pelvic and pectoral enlargements 

 and to have disappeared in between. 



Fig. 66.—Pleuracanthus ducheni, restored. A', ventral fin; B, basal fin- 

 cartUages; D, dermal margin of fin; DS., dermal fin-spine; H. A, htemal arches; 

 HMf hyo-mandibular; /. C, inter-neural plates; M. C, Meckel's cartilage; N, 

 notochord; NA, neural process and spine; P, supposed pelvic cartilage; PQ, 

 palatoquadrate; R, radial fin-cartilages; /J', ribs; S.G, shoulder girdle. (From 

 Parker and Haswell, after Dean.) 



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