126 THE ANATOMY OF VEETEBRATED ANIMALS. 



fishes, sucli as the Cod. No fossil Marsipohranchii are 

 known. This circumstance may, in part, be due to the 

 perishableness of their bodies ; though homy teeth, like 

 those of the Lampreys, might have been preserved under 

 favourable circumstances. 



III. The Elasmobranchii. — This order contains the 

 Sharks, the Rays, and the Chimcera. 



The integument may be naked, and it never possesses 

 scales like those of ordinary fishes ; but, very commonly, it 

 is developed into papillaj, which become calcified, and give 

 rise to toothlike structures : these, when they ai-e very 

 small and close-set, constitute what is called shagreen. When 

 larger and more scattered, they form dermal plates or 

 tubercles ; and when, as in many cases, they take the form 

 of spines, these are called dermal defences, and. in a fossil 

 state, ichthyodorulites. All these constitute what has been 

 called a "placoid exosheleton " and, in minute structure, 

 they precisely resemble teeth, as has been already explained. 

 The protruded surfaces of the dermal defences are fre- 

 quently ornamented with an elegant sculpturing, which 

 ceases upon that part of the defence which is imbedded 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 

 developed in its walls, to complete vertebrae, with deep 

 conical anterior and posterior concavities in their centra 

 and having the primitive cartilage more or less completely 

 replaced by concentric, or radiating, lamellse of bone. In 

 the Rays, indeed, the ossification goes so far as to convert 

 the anterior part of the vertebral column into one con- 

 tinuous bony mass. 



The neural arches are sometimes twice as numerous as 

 the centra of the vertebra, in which case the added arches 

 are termed inter crv/ral cartilages. 



