72 LOWER VERTEBRATES. 



studied the brain, and regards it as combining Selachian, Ganoid, and Batrachian char- 

 acters. Other anatomical features are similarly generalized, and these facts all lead 

 to the conclusion that the Holocephali are the modern I'epresentatives of an ancestral 

 type, closely connected with the other Elasmobraiichs and with the Ganoids, the 

 Dipnoans, and the Batrachians as well. The eggs are enveloped in horny capsules, 

 but nothing of importance is as yet known of the development. 



The earliest remains, which, without doubt, belong to the Holocephali, appear in 

 rocks of Jurassic age, but Dr. Newberry thinks the genus lihynchoches from the Devo- 

 nian of Ohio should be placed here. In many cases it is impossible to decide whether 

 a given fossil belongs to the Holocephali or to the Dipnoans. 



The sub-class Holocephali, as represented in the existing seas, is coextensive with 

 the family CniM^EiD.E. This is represented by two genera; (7/ /)»«■)■«, mostly from 

 the northern seas, and Callor/njnchus, from those of the Antarctic region. Of the 

 former, four species are recognized. Best known is C monstrosas, the chimajra, sea- 

 cat, sea-rat, king-fish, spook, etc., of fishermen. This is found mostly in the Arctic 

 seas, but it occurs in the Mediterranean and Jajian, and is said to have been taken at 

 the Cape of Good Hope. On our east coast occurs C. ji7??M«5ea., but it is "\-cry rare- 

 C. colliei of the Pacific is abundant from Monterey north. Xone of these have any 

 economic value, as the flesh is not good. 



The single species of Callorhynclius is found in the Antarctic seas, and from this 

 fact derives its specific name antarctlcvs. The most interesting feature connected 

 Avith it are its peculiar eggs. These are broad and flattened, and reach a length of ten 

 inches, with a width of about three ; their shape and their dark-green color make 

 them resemble some of the broad-fronded green algffi. On one side of this capsule are 

 two slits, which allow entrance and exit of water, for respiratory purposes. The epgs 

 are buried in the sand, a little below low-water mark. In the embrjos found by Pro- 

 fessor T. Jeffrey Parker, the yolk sac was covered with numerous paired blunt projec- 

 tions, and there was practically no yolk stalk. 



Sub-Class II. — Plagiostomi. 



The sub-class Plagiostomi, or Selachii, as it is variously termed, embraces the sharks 

 and ra3's, and hence by far the greater jDortion of Elasmobranchs. Among the 

 characters common to all the members, we may mention the following. The skin is 

 rarely naked, but is usually shagreened or covered with larger bony jilates or shields, 

 which sometimes may be developed into spines. The nostrils and the mouth are on 

 the lower surface of the head, and the latter is transverse, and usually armed with 

 numerous teeth. The palato-quadrate (upper jaw) and suspensorium are separate 

 and movably articulated with the skull. The spiracle (first visceral cleft) frequently 

 persists through life ; the gill-clefts are usually five in number, but may be six or seven; 

 they are never covered by an operculum. The vertebra; are well developed, and occa- 

 sionally a diplospondylous condition occurs, each body bearing two arches. 



Order I. — SQUALL 



In popular estimation sharks have an unenviable name. They are regarded, and 

 justly so, as the most cruel, rapacious, and bloodthirsty of all the inhabitants of the sea. 

 All of the known species, with four exceptions, are carnivorous, and while the smaller 

 species make sad havoc with crabs, molluscs, and schools of fish, the larger ones, when 



