182 The Class Elasmobranchii or Shark-like Fishes 
mouth); Chondropterygit (yovOpos, cartilage; wrepv6, fin); and 
Antacea (avraxaios, sturgeon). They represent the most 
primitive known type of jaw-bearing vertebrates, or Gnatho- 
stom (yvados, jaw; croua, mouth), the Chordates without jaws 
being sometimes called collectively Agnatha (a-yvados, without 
jaws). These higher types of fishes have been also called 
collectively Lyrifera, the form of the two shoulder-girdles taken 
together being compared to that of a lyre. Through shark- 
like forms all the higher vertebrates must probably trace their 
descent. Sharks’ teeth and fin-spines are found in all rocks 
from the Upper Silurian deposits to the present time, and while 
the majority of the genera are now extinct, the class has 
had a vigorous representation in all the seas, later Paleozoic, 
Mesozoic, and Cenozoic, as well as in recent times. 
Most of the Elasmobranchs are large, coarse-fleshed, active 
animals feeding on fishes, hunting down their prey through 
superior strength and activity. But to this there are many 
exceptions, and the highly specialized modern shark of the 
type of the mackerel-shark or man-eater is by no means a fair 
type of the whole great class, some of the earliest types being 
diminutive, feeble, and toothless. 
Subclasses of Elasmobranchs.—With the very earliest recog- 
nizable remains it is clear that the Elasmobranchs are already 
divided into two great divisions, the sharks and the Chimeras. 
These groups we may call subclasses, the Selachit and the Holo- 
cephali, or Chismopnea. 
The Selachit, or sharks and rays, have the skull hyostylic, 
that is, with the quadrate bone grown fast to the palate which 
forms the upper jaw, the hyomandibular, acting as suspen- 
sorium to the lower jaw, being articulated directly to it. 
The palato-quadrate apparatus, the front of which forms 
the upper jaw in the shark, is not fused to the cranium, although 
it is sometimes articulated with it. There are as many external 
gill-slits as there are gill-arches (5, 6, or 7), and the gills are 
adnate to the flesh of their own arches, without free tips. The 
cerebral hemispheres are grown together. The teeth are sepa- 
rated and usually strongly specialized, being primitively modified 
from the prickles or other defences of the skin. There is no 
frontal holder or bony hook on the forehead of the male. 
