514 The Class Elasmobranchii or Shark-like Fishes 



the other hand, the relation of the Chimaeras to the sharks is 

 stiU far from clearly understood. 



Order Pleuropterygii. — The order of Pleiiropterygii of Dean 

 {nXevpov, side; nrepvB,, fin), called by Parker and Hasweh Clado- 

 sdaclica, consists of sharks in which the pectoral and ventral 

 fins have each a very wide horizontal base fptychopterygium), 

 without jointed axis and without spine. There are no spines 

 in any of the fins. The dorsal fin is low, and there were proba- 

 bly two of them. The notochord is persistent, without inter- 

 calary cartilage, such as appear in the higher sharks. The 

 caudal fin is short, broad, and strongly heterocercal. Appar- 

 ently the ventral fin is without claspers. The gill-openings were 

 probably covered by a dermal fold. The teeth are weak, 

 being modified denticles from the asperities of the skin. The 

 lateral line is represented by an open groove. The family of 

 Cladoselachidw consists of a single genus Cladoselachc from the 

 Cleveland shale or Middle Devonian of Ohio. Cladoscladic jyleri 

 is the best-known species, reaching a length of about two feet. 

 Dean regards this as the most primitive of the sharks, and the 

 position of the pectorals and ventrals certainly lend weight 

 to Balfour's theory that they were originally derived from a 

 lateral fold of skin. I am recently informed by Dr. Dean that 

 he has considerable evidence that in Cladoselache the anus 

 was subtcrminal. If this statement is verified, it would go far 

 to establish the primitive character of Cladoselache. 



Order Acanthodei. — Near the Pleuropterygii, although much 

 more highly developed, we may note the strange group of Acan- 



FiG. 300. — Cladoselache fi/lcri (Newberry"), restored. Upper Devonian of Ohio. 



(After" Dean.) 



tliodei {aKavdoS?/^, spinous) . These armed fishes were once placed 

 among the Crossopterygians, but there seems no doubt that 

 Woodward is right in regarding them as a highly specialized aber- 

 rant offshoot of the primitive sharks. In this group the pairea 



