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University of California Publications in Geology [Vol. 7 



The character of the dentition in the lower jaw specimen, no. 

 19764, does not agree fully with any of the described genera. 

 It is intermediate between Hypohippus and Parahippus, and 

 evidently approximates Archacohippus in many respects. So 

 far as the stage of evolution is concerned, the Mohave form would 

 seem to come fairly near Parahippus. It shows elongation of the 

 crown, the lateral cingnlum is reduced, and there is clearly 

 defined incipient division of the metaconid and metastylid pillars. 



Measurements Archaeo- 

 hippus 



ultimus 



No. 19764 No. 1700 



Length, anterior side of P., to posterior side of M 2 — . 60 mm. 



Po, approximate anteroposterior diameter 16 11.5 



P 3 , approximate anteroposterior diameter 15.8 12 



P 3 , transverse diameter across hypoconid 10.5 9.8 



P 4 , anteroposterior diameter 15 



P 4 , transverse diameter across hypoconid 10.5 



M„ greatest anteroposterior diameter 13.6 



M,, transverse diameter across protoconid 9 



M 2 , greatest anteroposterior diameter 13.6 



M,, transverse diameter across protoconid 8.5 



Relationships. — The upper and lower jaw specimens (nos. 

 19840 and 19764) from the Mohave region resemble each other 

 in a number of important particulars. Their similarity in struc- 

 ture, and their occurrence in the same region give a reasonable 

 assurance that they represent the same type. The two speci- 

 mens show similarity in the following characters: (1) height of 

 tooth crowns; (2) rugosity of enamel; (3) absence of cingulum 

 on the protocone side; (4) stage of development, as seen in sep- 

 aration of metaconid and metastylid, in increase of size and 

 compression of the protoconule, in complication of the metaloph, 

 and in increase of size in hypostyle. The stage of evolution in 

 the two specimens shows about equal advance beyond the 

 dentition of Hypohippus. 



As has been suggested for the two specimens considered 

 separately, the form represented by them shows resemblance to 

 Hypohippus, Archaeohippus, Parahippus, and Anchitherium. It 

 is in general more advanced than Hypohippus. Its habit and 



