112 THE ANATOMY OF YERTEBRATED ANIMALS. 



and, in minute structure, they precisely resemble teeth, as has 

 been already explained. The protruded surfaces of the dermal 

 defences are frequently ornamented with an elegant sculptur- 

 ino", which ceases upon that part of the defence which is im- 

 bedded in the skin. The dermal defences are usually implanted 

 in front of the dorsal fins, but may be attached to the tail, or, 

 in rare cases, lie in front of the paired fins. 



The spinal column exhibits a great diversity of structure : 

 from a persistent notochord exhibiting little advance upon that 

 of the Marsipohrancliii^ or having mere osseous rings devel- 

 oped in its walls, to complete vertebrae, with deep conical an- 

 terior and posterior concavities in their centra, and having the 

 primitive cartilage more or less completely replaced by concen- 

 tric, or radiating, lamella3 of bone. In the Rays, indeed, the 

 ossification goes so far as to convert the anterior part of the 

 vertebral column into one continuous bony mass. 



The neural arches are sometimes twice as numerous as the 

 centra of the vertebrae, in which case the added arches are 

 termed intercrural cartilages. 



The terminal part of the notochord is never enclosed with- 

 in a continuous bony sheath, or urostyle. The extremity of 

 the vertebral column is generally bent up, and the median fin- 

 rays which lie below it are, usually, much longer than those 

 wiiich 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 heterocercal^ while those in which 

 the fin-rays of the tail are equally divided by the spinal col 

 umn, or nearly so, are diphy cereal (p. 21). The Monkfish 

 {Squatina) and many other Ekismohranchii are more diphy- 

 cercal than heterocercal. 



The ribs are always small, and may be quite rudimentary. 



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 movable 

 upon the spinal column, it articulates therewith by two con- 

 dyles. 



In its general form and structure, the cartilaginous skull 

 of an Elasmohranch corresponds with the skull of the verte- 

 brate foetus in its cartilaginous state, and there are usually 

 more or less extensive membranous fontanelles in its upper 

 walls. The ethmoidal region sends horizontal plates over the 

 nasal sacs, the apertures of which retain their embryonic situ- 

 ation upon the under-surface of the skull. 



Neither premaxillae nor maxillae are present, the *' jaws ^' 



