78 



University of California Publications in Geology [Vol. 10 



The horses of the Phillips Ranch fauna are of a type in or near 

 the genus Merychippus, but possibly represent a new subgenus of 

 that group less advanced in tooth structure than typical Merychippus. 

 They are represented by upper and lower cheek-teeth and by a number 

 of limb bones. The upper cheek-teeth are short hypsodont, the height 

 being about equal to or a little greater than the transverse diameter 

 in well worn teeth. The teeth are small, those in the middle of the 

 series ranging from 15.6 to 17.1 mm. in anteroposterior diameter and 

 from about 15 to 16.4 nun. transverse diameter. Height of worn teeth 

 on the outer side is 15 to 18 mm., on inner side 10.2 to 12.2 m. They 

 are relatively narrow transversely compared with the teeth of typical 

 species of Merychippus. The outer and inner faces are not approxi- 

 mately parallel as in strictly prismatic teeth, but approach each other 

 somewhat toward the summits of the teeth. The crowns are slightly 

 curved. They are well cemented, but the cement covering is not heavy. 

 The outer faces of paracone and metacone are flat or have a very faint 

 median rib. The mesostyle is prominent, narrow, and may be of nearly 

 equal width from base to summit of tooth, or may flare slightly at the 

 base. The protoconule and metaconule are slightly larger than proto- 

 cone and hypocone. The protoconule is slightly crescentic and is 

 broadly connected with the oval protocone. The metaloph is com- 

 pletely connected with ectoloph, but is usually not connected with 

 protoloph because of imperfect development of the posterior horn of 

 the protoconule. The fossettes are well cemented, with one or two 

 crinkles in the enamel borders. The prefossette is usually open on the 

 median or inner border between protoloph and metaloph. The post- 

 fossette is sometimes closed and sometimes open between bypocone 

 and hypostyle. 



Tbe upper cheek-teeth are more advanced than those of Parahippus 

 and the other genera of the Anchitheriinae in the greater height of 

 the crowns and in their prismatic form, in their strong cementation, 

 in the relatively large size of protoconule and metaconule compared 

 with protocone and hypocone, and in the tendency to close the fos- 

 settes. They are relatively primitive compared with described forms 

 of Merychippus in their small size, shorter crowns, subequality in size 

 of protocone and protoconule, in the imperfect development of the 

 posterior horn of the protoconule, and in the general openness of the 

 fossettes on the inner side. 



The characters are in a measure those assigned by Gidley 2 to 

 hypothetical genera of a group "C" of division "Anchitheriinae 



2 Gidley, J. W., Bull. Am. Mus. Nat. Hist., vol. 23, p. 868, 1907. 



