UQ 



Berry and Gregory — Prorosmarus alleni. 



the second and third "molars" is a prominent rounded protu- 

 berance about 4 cm long and gradually fading away posteriorly. 

 This protuberance may have furnished attachment for liga- 

 ments binding the rami together as well as for the digastric 

 muscle which depressed the jaw. In the modern walrus this 

 process is indicated in the young jaw, but becomes strongly 

 inflected, lengthened, and less conspicuous in the adult. Addi- 

 tional and less important differences from Odobcenus are as 



Fig. 3. Type lower jaw (right ramus) of Alachtherium cretsii DuBus. 

 Internal view, one-fourth natural size. (After Van Beneden.) 



follows : (9) The subequal size of the molariform teeth which 

 in Odobamus successively decrease in size, the most anterior 

 (c) being the largest. (10) The molars must have been circular 

 in section whereas in the adult walrus they are laterally com- 

 pressed. The molariform teeth probably also had a somewhat 

 more pointed tip than in Odobcenus. (11) Prorosmarus 

 retained one more fully functional molariform tooth (j? 4 ), which 

 is vestigial or absent in the walrus. 



In brief the jaw of Prorosmarus seems to be much less 

 specialized than is the modern walrus jaw. In the latter, 

 doubtless in response to the need for an effective crushing appa- 

 ratus, the distal portion of the jaw has become very massive ; 

 the enormous growth of the upper canines has caused the dis- 

 appearance of the incisors and the transference of the lateral 

 upper incisor and the lower canine to the molariform or cheek 

 series ; the effective center of the lower molar series has 

 gradually been shifted forward and the upper molar series 

 have acquired oblique wearing surfaces, so that the shock of 

 impact is partially transmitted to the massive upper canines. 



