NO. 5 TELESCOPING OF THE CETACEAN SKULL 29 



ported by transverse processes which ordinarily, in Hving forms at 

 least, have no obvious peculiarities making them appear to be not 

 serially homologous with those which support the others. Normally 

 the intermaxillary does not extend forward much beyond the maxil- 

 lary, but in the extinct eurhinodelphines it is said to be so greatly pro- 

 duced as to form about one-third of the excessively attenuate beak 

 (I do not regard the published evidence as conclusive; see pp. 49-50). 

 The teeth are of two types. In the more usual form, found in most 

 of the Delphinidce except Delphinapterns and Mondon, their struc- 

 ture and method of growth is like that of simple conical teeth in other 

 mammals ; that is, the crown consists of a dentine shaft capped with 

 enamel and with or without a deposit of cement on the lower portion ; 

 the pulp cavity closes when the tooth has reached its full size, and 

 the crown gradually wears down to the level of the gum. In the 

 other form their structure and method of growth are tusk-like ; that is, 

 the enamel cap is so reduced as to be visible in very young teeth only, 

 and to be of no functional importance at any time,^ while the cement 

 is so increased that it becomes a conspicuous mechanical portion of 

 the shaft of the tooth ; the pulp cavity remains open throughout much 

 or all of the animal's life, and the wearing away of the crown is con- 

 tinually compensated for by new growth from below. This tusk- 

 like type of growth occurs in its typical condition in Delphinapterns 

 (described and illustrated by Lonnberg, Arkiv for Zoologi, Vol. 7, 

 No. 2, July 5, 1910), where each moderate sized functional tooth in 

 the adult is the constantly worn down base of a tusk, which, if entire, 

 would be not less than 120 mm. in length when fully grown. The 

 enormously enlarged tusk of the male Monodon appears to be a 

 development from a tooth of this kind. Modifications of an essen- 

 tially similar tusk-like condition are found in the beaked whales as 

 well as in the more distantly related Physeteridce and Kogiidco, and 

 in several fossils of doubtful affinity. 



Though the toothed cetacean type which seems to be the most 

 efficient has been developed by those members of the group in which 

 the maxillary bone passes horizontally backward over the entire 



' Flower says of some young sperm whale teeth ahout 40 mm. in length : 

 " . . . . they show no trace of an enamel covering to the apex, a point which 

 has hitherto been one of uncertainty" (Trans. Zool. Soc. London, Vol. 6, 

 p. 325). Teeth 10 mm. and 15 mm. long in the U. S. National Museum 

 (No. 49488) show irregular patches of a substance which appears to be 

 enamel at the extreme tip and scattered over the rapidly tapering terminal 

 half of the crown. These patches probably represent the last remnants of the 

 degenerating enamel cap. 



