TISSUE OF TOOTH IN FISHES. 247 



Hard or true dentine — Sparoids, labroids, lopliius, 

 balistes, pycnodonts, prionodon, sphyrcena, megalichthjs, 

 rhizodes, diodon, scarus ; 



Osteodentine — Cestracion, acrodus, lepidosiren, cteno- 

 dus, hybodus, percoids, sciaenoids, cottoids, gobioids, 

 sharks, and many others ; 



Yasodentine — Psammodus, chimteroids, pristis, mylio- 

 bates ; 



Plicidentine — Lophius, holoptychius, bothriolepis ; and 



Dendrodentine — Dendrodus ; 



Besides the compound teeth of the scarus and diodon. 



One structural modification may prevail in some teeth, 

 another in other teeth, of the same fish ; and two or more 

 modifications may be present in the same tooth, arising 

 from changes in the process of calcification and a persist- 

 ency of portions or processes of the primitive vascular 

 pulp or matrix of the dentine. 



The dense covering of the beak-like jaws of the parrot- 

 fishes {Scari) consists of a stratum of prismatic denticles, 

 standing almost vertically to the external surface of the 

 jaw bone ; this peculiar armature of the ja^vs is adapted 

 to the habits and exigences of a tribe of fishes which 

 browse upon the lithophytes that clothe, as with a richly 

 tinted carpet, the bottom of the sea, just as the ruminant 

 quadrupeds crop the herbage of the dry land. 

 . The irritable bodies of the gelatinous polypes, which 

 constitute the food of these fishes, retreat, when touched, 

 into their star-shaped stony shells, and the scari conse- 

 quently require a dental apparatus strong enough to 

 break off or scoop out these calcareous recesses. The 

 jaws are, therefore, prominent, short, and stout, and the 

 exposed portions of the premaxillaries and premandibu- 

 lars are incased by a complicated dental covering. The 



