CHAPTEE XVII 



ELASMOBEANCHII : GENERAL CHAEACTEES PLEUEOPTERYGII 



ICHTHYOTOMI ACANTHODEI PLAGIOSTOMI SELACHII 



BATOIDEI HOLOCEPHALI 



CLASS II. PISCES. 



Sub-Class I. Elasmobranchii. 



In both the ancient and the modern Sharks, Dog-Fishes, and 

 Eays, the exoskeleton takes the form of a more or less uniform 

 investment of dermal denticles or " shagreen." The endo- 

 skeleton is wholfy cartilaginous or partially calcified, and there 

 are neither cartilage- nor membrane-bones. The vertebral column 

 is acentrous or chordacentrous, generally with alternating basi- 

 and inter-dorsal elements, and terminating in a heterocercal tail. 

 The skull is usually hyostylic, very rarely amphistylic or auto- 

 stylic, and the lateral halves of the primary upper jaw (palato- 

 quadrate cartilages) usually meet in a highly characteristic 

 median symphysis beneath the base of the skull. Branchial 

 arches and clefts are five to seven in number, and the clefts are 

 separated by complete interbranchial septa, which, as a rule, are 

 continuous externally with the skin. An operculum is developed 

 only in the Holocephali. A pelvic girdle is present. With rare 

 exceptions the pectoral fin is uniserial. The pelvic fin is in- 

 variably uniserial. The exoskeletal supports of all the fins 

 consist of ceratotrichia, and, when present, the fin-spines are 

 invested by enamel. Claspers are generally present in the males. 

 In the surviving members of the group the nostrils retain 

 their primitive ventral position. There is a conus arteriosus 

 with several rows of valves. A spiracle, often furnished with a 

 spiracular pseudobranch, is generally present, and, as a rule, 



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