The Class Elasmobranchii or Shark-like Fishes 507 



not of several pieces, but of a cartilage called Meckel's cartilage, 

 which in higher fishes precedes the development of a separate 

 dentary bone. These structures are sometimes called primary 

 jaws, as distinguished from secondary jaws or true jaws de- 

 veloped in addition to those bones in the Actiiioptcri or typical 

 fishes. In the sharks the shoulder-girdle is attached, not to 

 the skull, but to a vertebra at some distance behind it, leaving 

 a distinct neck, such as is possessed or retained by the verte- 

 brate higher than fishes. The shoulder-girdle itself is a con- 

 tinuous arch of cartilage, joining its fellow at the breast of the 

 fish. Other peculiar traits will be mentioned later. 



Characters of Elasmobranchs. — The essential character of the 

 Elasmobranchs as a whole are these : The skeleton is cartilagi- 

 nous, the skull without sutures, and the notochord more or 

 less fully replaced or inclosed by vertebral segments. The jaws 

 are peculiar in structure, as are also the teeth, which are usually 

 highly specialized and found on the jaws only. There are no 

 membrane bones; the shoulder-girdle is well developed, each 

 half of one piece of cartilage, and the ventral fins, with the 

 pelvic-girdle, are always present, always many-rayed, and 

 abdominal in position. The skin is covered with placoid scales, 

 or shagreen, or with bony bucklers, or else it is naked. It is 

 never provided with imbricated scales. The tail is diphycercal, 

 heterocercal, or else it degenerates into a whip-like organ, a 

 form which has been called leptocercal. The gill-arches are 5, 

 6, or 7 in number, with often an accessory gill-slit or spiracle. 

 The ventral fins in the males (except perhaps in certain primi- 

 tive forms) are provided with elaborate cartilaginous appen- 

 dages or claspers. The brain is elongate, its parts well separated, 

 the optic nerves interlacing. The heart has a contractile 

 arterial cone containing several rows of valves; the intestine 

 has a spiral valve ; the eggs are large, hatched within the body, 

 or else deposited in a leathery case. 



Classification of Elasmobranchs. — The group of sharks and 

 their ahies, rays, and ChimJEras, is usuahy known collective^ as 

 Elasmobranchii (elaa-z-iog, blade or plate; /3pdyx"S, giU)- Othcr 

 names apphed to ah or a part of this group are these; Sdachii 

 (o-eAajos, a cartilage, the name also used by the Greeks for the 

 gristle-fishes or sharks); Plagiostomi (nXayw?, obhque; (jTOjia, 



