SHARKS. 



27 



flection of the mucous membrane of the mouth, which would be 

 lacerated by such a movement ; it is by a gradual change of position 

 in the fibrous membrane to which their base is attached, that the 

 altered direction of the consolidated teeth is effected. 



The teeth present the smallest relative size among the sharks, 

 in the sub-genus Rhinodon of Dr. Smith, where they may be com- 

 pared with the teeth en brosse, of certain osseous fishes ; here they 

 are of a simple conical, slightly recurved form ; there are twelve or 

 thirteen teeth in each vertical row, and about two hundred and fifty 

 of such rows in each jaw. 



In the sub-genus Selache, to which the great basking shark, 

 {Squalus maximus, Home), belongs, the teeth, though small, are rela- 

 tively larger than in jR/imocZon. They are conical, recurved, and with a 

 somewhat obtuse apex. In a specimen about thirty-six feet in length, 

 the teeth, which are alike in both jaws, measure not quite half an 

 inch in length, and between two and three lines across their rounded 

 base. The sharks with teeth of larger size and more formidable 

 aspect present many modifications of shape, by which, with other 

 characters, the genera and sub-genera of squaloids are distinguishable. 

 The principal varieties of form are illustrated in plates 3 and 4. Varieties 

 of form, however, it should be remembered, are not only indicative of 

 generic distinctions, but sometimes of a difference of age of the same 

 individual. In the common spotted dog-fish, for example, the one 

 or two lateral denticles at the base of the principal cusp described 

 as characteristic of the teeth of the genus Scyllium, frequently disap- 

 pear in the old fishes. In the young of the blue-shark (Carcharias 

 glaucus) the teeth have smooth trenchant edges, but in the old ones, 

 the margins are dentated. In many genera, again, the teeth of the 

 upper differ in form from those of the lower jaw ; this is most re- 

 markably the case in the genus Scymnus : they also frequently differ 

 in shape as well as in size in different parts of the same jaw. But 

 while a knowledge of these facts should impress the observer with 

 due caution in giving an opinion on the specific or generic relations 

 of an extinct shark from the examination of a single tooth, the pe- 

 culiarities of form characteristic of different genera of existing squa- 

 loids are sufiiciently constant and well marked to render dental 



