508 The Class Elasmobranchii or Shark-like Fishes 



mouth) ; Chondropterygii (xovdpos, cartilage ; Trrept'C, fin) ; and 

 Antacea (avraKaios, sturgeon). They represent the most 

 primitive known type of jaw-bearing vertebrates, or Gnatho- 

 stomi (yvdOos, jaw; crro^a, mouth), the Chordates without jaws 

 being sometimes called collectively Agnatha (d-yvd6o$, 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 Palaeozoic, 

 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 Chim&ras. 

 These groups we may call subclasses, the Selachii and the Holo- 

 cephali, or Chismopnea. 



The Selachii, 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. 



