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University of California Publications. [Geology 



which are generally closely aggregated and nearly circular in 

 cross-section. In some instances they are closely grouped and 

 become angulated where they are in contact. The pillars are 

 generally arranged in pairs set transverse to the longest diameter 

 of the crushing face. In some cases three pillars are present 

 in the transverse row. The average tooth comprises three pairs 

 of pillars. In one specimen four transverse 

 rows are present. In section (figs 15 and 

 3) the pillars are seen to consist of an ex- 

 traordinary thick enamel layer and a com- 

 paratively small dentine body. The enamel 

 resembles in general characters that of the 

 teeth in members of the mastodon group. 

 In wear the pillars usually show a very 

 thick rim of enamel surrounding a small 

 central pit worn into the softer dentine. 

 With wear the size of the central dentine 

 area increases until, in a half-worn tooth, 

 its diameter may about equal the thickness 

 of the enamel ring. In the field the teeth 

 are most commonly found broken up into 

 fragments of pillars. 



Of the known Japanese specimens very 

 fortunately one includes a large part of a 

 skull with a number of teeth in the jaws. In 

 this individual there are three cheek-teeth 

 shown in the upper jaws (figs. 4 and Jl) 

 and three in the lower (figs. 5 to 7). The 

 anterior tooth in each jaw is much smaller 

 than the tooth immediately behind it, and was considered by 

 Yoshiwara and Iwasaki as P 4 in the upper jaw and P :1 in the 

 lower jaw. The anterior cheek-tooth, P*, of the upper jaw con- 

 sists of four pillars of which the posterior pair are relatively 

 quite small. The second upper cheek-tooth, M 1 , is at least three 

 times as large as P 4 . It consists of eight large pillars, of which 

 three form the anterior transverse row, two pairs form the second 

 and third transverse rows, and a single pillar forms the posterior 



Fig. 3. Desmostylus, 

 sp. Fragment of a 

 cheek-tooth showing 

 thickness of enamel. 

 No. 10015, natural size. 

 Lower Miocene"?, near 

 Santa Ana, California. 

 Collected by C. H. 

 McCharles. 



