The Class Elasmobranchii or Shark-like Fishes 181 
not of several pieces, but of a cartilage called Meckei’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 Actinopteri 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 5s, 
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 allies, rays, and Chimezeras, is usually known collective y as 
Elasmobranchii (éhac os, blade or plate; Bpayyos, gill). Other 
names applied to all or a part of this group are these: Selachii 
(ceAa@yos, a cartilage, the name also used by the Greeks for the 
gristle-fishes or sharks); Plagiostomi (xlayios, oblique; croya, 
