322 PHILOSOPHICAL TRANSACTIONS. [anNO J 787. 



most of this tribe is well adapted for catching the food ; the jaws spread as they 

 go back, making the mouth proportionally wider than in many other animals. 

 There is a very great variety in the formation of the mouths of this tribe of 

 animals, which we have many opportunities of knowing, from the head being 

 often brought home when the other parts of the animal are rejected ; a circum- 

 stance which frequently leaves us ignorant of the particular species to which it 

 belonged. 



Some catch their food by means of teeth, which are in both jaws, as the por- 

 poise and grampus ; in others, they are only in one jaw, as in the spermaceti 

 whale ; and in the large bottle-nose whale, described by Dale, there are only 2 

 small teeth in the anterior part of the lower jaw. In the Narwhale only 2 tusks 

 in the fore part of the upper jaw ; * while in some others there are none at all. 

 In those which have teeth in both jaws, the number in each varies considerably; 

 the small bottle-nose had 46 in the upper, and 50 in the lower ; and in the 

 jaws of others there are only 5 or 6 in each. 



The teeth are not divisible into different classes, as in quadrupeds ; but are 

 all pointed teeth, and are commonly a good deal similar. Each tooth is a double 

 cone, one point being fastened in the gum, the other projecting: they are 

 however not all exactly of this shape. In some species of porpoise the fang is 

 flattened, and thin at its extremity ; in the spermaceti whale the body of the 

 tooth is a little curved towards the back part of the mouth ; this is also the case 

 in some others. The teeth are composed of animal substance and earth, simi- 

 lar to the bony part of the teeth in quadrupeds. The upper teeth are com- 

 monly worn down on the inside, the lower on the outside ; this arises from the 

 upper jaw being in general the larger. 



The situation of the teeth when first formed, and their progress afterwards, 

 as far as I have been able to observe, is very different in common from those of 

 the quadruped. In the quadruped the teeth are formed in the jaw, almost sur- 

 rounded by the alveoli, or sockets, and rise in the jaw as they increase in length; 

 the covering of the alveoli being absorbed, the alveoli afterwards rise with the 

 teeth, covering the whole fang ; but in this tribe the teeth appear to form in the 

 gum, on the edge of the jaw, and they either sink in the jaw as they lengthen, 

 or the alveoli rise to inclose them : this last is most probable, since the depth of 

 the jaw is also increased, so that the teeth appear to sink deeper and deeper in 

 the jaw. This formation is readily discovered in jaws not full grown ; for the 

 teeth increase in number as the jaw lengthens, as in other animals. The pos- 



* I call these tusks to distinguish them from common teeth. A tusk is that kind of tooth which 

 has no bounds set to its growth, excepting by abrasion, as the tusk of the elephant, boar, sea-horse, 

 raanatee, &c. — Orig. 



