1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 545 



ISCHYEOSMILUS OSBOENI, n. sp. 



Type specimen, a portion of a left lower jaw with dentition, no. 19476, Univ. 

 Calif. Col. Vert. Palae. From the Ricardo beds near Eed Eock Canon, California. 



The type specimen (fig. 155) represents a species most nearly 

 related to Ischyrosmilus ischyrus from McKittrick, on the western 

 side of the San Joaquin Valley, California. The specimen of 

 I. osborni, represents the same portion of the mandible shown in the 

 type of I. ischyrus, so that a close comparison can be made. This 

 species differs from I. ischyrus in its smaller size, somewhat flatter 

 anterior face of the symphysial region, much thinner or transversely 

 narrower cheek-teeth, and in the division of the posterior lobe of P 4 

 into two cusps. 



The anterior end of the masseteric fossa in the specimen of 

 I. osborni is much narrower vertically than in the type of /. ischyrus, 

 and suggests that the form of the jaw in this region may be found 

 to be different in the two species. 



The incisors have lost the greater part of their crowns, but seem 

 smaller and narrower transversely than in I. ischyrus. The canine 

 is not represented. 



P 3 is known only by the root witli a small portion of the crown. 

 The root is small and round in cross-section, with no suggestion of 

 division. 



P 4 resembles the corresponding tooth of I. ischyrus, excepting in 

 the slightly larger size of the posterior lobe, and in its beginning 

 division into two cusps. 



Comparative Measurements 



I. ischyrus 

 no. 8140 

 Tulare? 



I. osborni 

 no. 19476 

 Ricardo 





Length from anterior side of L to posterior side of M,.~ . 



123 mm. 



101 



Length from anterior side of canine to posterior side of M, 



107 



81 



Width of anterior face of symphysial region 



52 



a36 



Depth of jaw across the middle of the flange 



58 



44 



Depth of mandible below posterior end of P 4 



37.5 



32.3 



Length of inferior diastema ! 



33.5 



26.5 



Width of I t transversely 



4 



2.5 



Width of I, transversely 



6.5 



4 



Anteroposterior diameter of inferior canine 



14.5 



alO 



Anteroposterior diameter of P a 



7.2 



a6.4 



Anteroposterior diameter of alveolus of P 



8.1 



7 



Anteroposterior diameter of P 4 



20 



16.7 



Anteroposterior diameter of Mj 



28.5 



24 



Transverse diameter of M x 



15 



11 



a, approximate. 



