THE ELASMOBEANCHII. 127 



The teiTainal part of the notochord is never enclosed 

 within a continuous bony sheath, or urostyle. The ex- 

 tremity of the vertebral column is generally bent up, and the 

 median fin-rays which lie below it are, usually, much longer 

 than those which lie above it, causing the lower lobe of the 

 tail to be much larger than the upper. Elasmobranchs with 

 tails of this conformation are truly lieterocercal, while those 

 in which the fin-rays of the tail are equally divided by the 

 s^jinal column, or nearly so, are diphycercal (p. 16). The 

 Monkfish {Sqiiatina) and many other Elasmobranchii are 

 more diphycercal than heterocercal. 



The ribs are always small, and may be quite nidimentai-y. 



The skull is composed of cartilage, in which superficial 

 pavement-like deposits of osseous tissue may take place, 

 but it is always devoid of membrane bone. When move- 

 able upon the spinal column, it articulates therewith by 

 two condyles. 



In its general form and structure, the cartilaginous skull 

 of an Elasmobranch corresponds with the skull of the verte- 

 brate fcetus in its cartilaginous state, and there are usually 

 more or less extensive membranous f ontanelles in its upper 

 walls. The ethmoidal region sends horizontal plates over 

 the nasal sacs, the apertui-es of which retain their em- 

 bryonic situation upon the under- surface of the skull. 



Neither premaxillse nor maxillae are pi-esent, the "jaws" 

 of an Elasmobranch consisting, exclusively, of cartilaginous 

 representatives of the primary palato-quadrate arch and of 

 Meckel's cartilage. 



The former of these, the so-called upper jaw, may either 

 be represented, as in the Chimcera (Fig. 33), by the anterior 

 portion {B, D) of a triangular cartilaginous lamella, which 

 stretches out from the sides of the base of the skull, and is 

 continuous with the representative of the hyomandibular 

 auspensortum; or there may be, on each side, a carti- 

 laginous bar moveably articulated in front with the fore 

 part of the skull ; and, posteriorly, funushing a condyle, 

 with which the ramus of the lower jaw, representing Meckel's 

 cartilage, articulates. 



