384 NATURAL SCIENCE. June, 



is as broadly expanded as in the palaeozoic Cladodus (Rose, 8, p. 200, 

 Fig. 7). Crushing teeth are not common in selachians, but are 

 found in the Port Jackson shark (Cestracion) (Fig. 10). In batoids, 

 flattened grinding teeth are of more frequent occurrence, and 

 the examination of the dentition of such a series of forms as Rata, 

 Rhynchobatus, Rhinoptem polyodon, Rhinoptera javanica, Myliobatis, and 

 Actobatis will explain the relation that exists between the single row 

 of bent dental bars in the latter genus and the more normal dentition 

 of Raia, which, in all essential respects, is the same as that of the 

 selachoid Scyllium. The crushing surface of the teeth of the 

 Cretaceous Ptychodus is marked by a series of parallel ridges, while 

 the margins are delicately granulate. A somewhat similar surface- 

 ornamentation is to be seen in the living Rhynchobatus, although here 

 the teeth are much smaller in size. 



Considerable variety of form is also found in the teeth of the 

 bony fishes. The commonest and most typical form of tooth is the 

 conical. Blunted, button-shaped teeth, frequently approaching the 

 hemispherical form, are found both in teleosteans, e.g., Pagms, and 

 the extinct lepidosteoid fishes, Lepidotus, Ccelodus, Gyrodus, etc. Blade- 

 shaped teeth occur in Sphyrana, and the teeth of the eel have the 

 form of compressed cones set transversely to the axis of the jaw. In 

 the chaetodont fishes the teeth are very long and slender, and in some 

 cases even flexible. 



The forms of teeth occurring in the Cyclostomata, Holocephali, 

 and Dipnoi are characteristic of these groups ; but, since they are 

 dealt with in other parts of this article, a special description of them 

 here is unnecessary. 



There are probably very few fishes in which all the teeth in the 

 mouth are exactly similar in character. In sharks, the teeth near the 

 symphysis of the jaws usually differ from those situated towards the 

 articular region in being larger and more powerful ; and those between 

 these extremes form a graduating series, in which the differences 

 between adjacent teeth are hardly perceptible. Not infrequently the 

 upper and lower teeth differ in shape, as in Notidanus (Fig. 3), 

 Carcharias (Fig. 4), and Lczmargus. Only in a few instances can the 

 term " heterodont " be applied to the dentition of fishes. In the Port 

 Jackson shark, Cestracion (Fig. 10), among Elasmobranchii, and in the 

 wolf-fish, Anarrhichas, among Teleostei, the front teeth are pointed 

 and adapted for the seizure of food, while the hinder are flattened into 

 powerful crushing organs. In such teleosteans as Chvysophvys and 

 Pagms, the hind teeth are more rounded than those in front, but the 

 difference is less marked than in Anarrhichas. Sexual differences in 

 the dentition are extremely rare, but striking instances occur in many 

 species of Raia, where the teeth are sharply pointed in the male, but 

 flattened in the female. 



Succession. — In sharks and rays the teeth are embedded in the 

 dense fibrous gum, and are arranged in rows parallel to the axis of 



