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University of California. 



[Vol. i. 



Figure i. — Sigmogomphius Le Contei. a, Skull, seen from above (natural 

 size). F, frontal; N, nasal; Pm, premaxillary ; M, maxillary; 

 I, incisor, b, right and left superior molar series (twice nat- 

 ural size). Right premolar shown in section (heavy line) a 

 short distance above the triturating surface. 



genus, it should perhaps be stated that, although the head bones 

 extend backward beyond the molars, no sign of a fourth tooth could 

 be found on either side. The striking resemblance of the molars to 

 those of Eucastor, Leidy, Fig. 2, a, which possibly possessed only 

 three superior molar teeth, makes it appear the more probable that 

 the fourth tooth was absent in Sigmogomphius. The molars were 

 semi-hypselodont or semi-prismatic and strongly curved. They 

 attained about half the size of those in Castor fiber, averaging near 

 i 1.5 mm. long; crowns, 4.8 mm. transversely, 4.5 mm. antero-poste- 

 riorly. Evidently they were not rooted till late in life, beginning to 

 close up on the outer posterior corners, at which place a small lateral 

 root was formed, the main portion continuing to grow for some time 

 after this separation. The crowns of the premolars were larger than 

 those of the molars, being about one-fourth longer and broader. 

 Molars one and two were of nearly equal size. The triturating sur- 

 face of the premolars is nearly triangular in outline; that of the 

 molars is more nearly quadrate. All are set obliquely in the jaw, 

 so that the folds of the enamel wall do not cut the fore-and-aft axis 

 of the tooth at right angles, but obliquely. 



The inner wall of all the molar teeth is thrown into a single 

 strong fold, which, in the first tooth, traverses half the breadth of 



