XIII] OF DOLPHINS' TEETH 899 



pursued its steady growth, uiiclianged otherwise in form, till it 

 attained the dimensions which we meet with in the mammoth. In a 

 species such as Mastodon angustidens, or M. arvernensis, the specific 

 angle is low and the tusk comparatively straight ; but the American 

 mastodons and the existing species of elephant have tusks which do 

 not differ appreciably, except in size, from the great spiral tusks of 

 the mammoth, though from their comparative shortness the spiral 

 is httle developed and only appears to the eye as a gentle curve. 

 Wherever the tooth is very long indeed, as in the mammoth or the 

 beaver, the effect of some slight and all but inevitable lateral 

 asymmetry in the rate of growth begins to shew itself: in other 

 words, the spiral is seen to lie not absolutely in a plane, but to be 

 a gauche curve, like a twisted horn. We see this condition very 

 well in the huge canine tusks of the babirussa; it is a conspicuous 

 feature in the mammoth, and it is more or less perceptible in any 

 large tusk of the ordinary elephants. 



The simplest of mammaUan teeth are, Uke those of reptiles, conical 

 bads which spring by single roots from a common origin: much as 

 the pinnules of a compound leaf spring from a common petiole. 

 A dolphin's teeth are typical of what .Cope* called a haplodont 

 dentition; a sloth's (whether degenerate or no) are no further 

 advanced ; canines remain unaltered throughout the mammaha, and 

 incisors vary little save for some flattening due to crowding in a 

 foreshortened jaw. Like the leaf and its pinnules, the tooth-germ 

 buds and branches in endless ways; and we have no criterion of 

 comparison (nor any right to expect it) between the individual 

 cusps of a dog's, an elephant's and a horse's teeth, any more than 

 between the several pinnules, cusps or leaflets of a rose, a maple 

 and a horse-chestnut. The tooth-buds remain apart or coalesce in 

 various numbers and degrees ; and croWding, abrasion and mechanical 

 pressure play a large part in the final arrangement and conformation f. 



The dolphin's teeth, used only for prehension, do not impinge 

 on one another, and stay sharp accordingly ; those of the carnivores 



* E. D. Cope, On the homologies and origin of the types of molar teeth in mam- 

 malian dentition, Pr. Ac. N. S. Philad. xxv, p. 371, 1873; Journ. Ac. N. S. Philad. 

 (c), vm, pp. 71-89, 1874. 



t Cf. J. A. Ryder, Mechanical genesis of tooth forms, Pr. Ac. N. S. Philad. 

 1878, pp. 45-80. 



