478 University of California Publications in Geology [Vol. 11 



The form represented by the lower jaw, no. 19764, shows a general 

 resemblance to Hypohippus, but differs in its slightly higher and more 

 rugose crowns, more clearly marked incipient separation of metaconid 

 and metastylid columns, and absence of external basal cingulum. 



This form differs from typical Parahippus in the very weak sep- 

 aration of the metaconid and metastylid columns, and in the absence 

 of cement from the crowns. The separation of metaconid and meta- 

 stylid in no. 19764 shows but little advance beyond the stage seen in 

 the dentition of a Hypohippus specimen from Virgin Valley. In none 

 of the cheek-teeth of no. 19764 are metaconid and metastylid pillars 

 separated on the inner side by more than a faint groove at the summit. 



Measurements Archaeo- 



hippus 

 ultimus 



No. 19764 No. 1700 



Length, anterior side of P-, to posterior side of 60 mm. 



P 2 , 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 



Mo, greatest anteroposterior diameter 13.6 



M 2 , transverse diameter across protoconid 8.5 



The upper and lower jaw specimens (nos. 19840 and 19764) from 

 the Barstow resemble each other in a number of important particulars. 

 Their similarity in structure and their occurrence in the same region 

 gave a reasonable assurance that they represent the same type. The 

 two specimens 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 separation 

 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 the Hypohippus. 



The Barstow type represented by specimens nos. 19840 and 19764 

 is evidently related to Parahippus in most characters, though distant 

 from the typical form. The absence of a crochet in the upper teeth, 

 and the very slight separation of metaconid and metastylid columns in 

 the lower teeth, indicate a relatively undeveloped stage. Whether this 

 form is too primitive to be included in Parahippus will be determined 

 more elearlv when better material is available for study. 



