374 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol. YI. 



on the grinding surface of the crown, Trhose long diameter is antero- 

 Ijosterior. 



The only lower jaw of a species of this genus in my possession presents 

 a small part of the base of the angle and of the coronoid process. These 

 parts are so nearly in the plane of the inci sive alveolus as to lead to 

 the belief that the genus Mylagaulus belongs to the suborder 8duro- 

 morplia. The rootless teeth with deep enamel fossettes approximates 

 it to the Castoridce, but it appears to me that a new family group must 

 be established for its reception.* Such characters are the presence of 

 only three inferior molars, and the entire independence of the enamel 

 fossettes of the external sheathing enamel. It is worthy of investigation 

 whether the Hystrix refossa Gerv. has any relation to this family. 



As a generic feature, the preponderance of the first true molar over 

 all the others is remarkable. It performed the greater part of the masti- 

 catory function, as the second molar is a small tooth, and the third one 

 quite insignificant, and in the ill. monodon probably early shed. 



The genus is only known from the Loui) Fork formation. There are 

 two species, both from the adjacent regions of Kansas and Nebraska. 



Mylagaulus monodon Cope. 



American Naturalist, 1881, July. 



The larger species, and represented by a left mandibular ramus, from 

 which the more fragile parts have been broken. The form of the ramus 

 is rather compressed and deep, and the line of molar teeth is very little 

 oblique to its plane. The diastema is short and concave, and the in- 

 cisive alveolar margin is elevated. The base of the coronoid i)rocess 

 originates opposite the middle of the second molar. The internal and 

 external faces of the ramus are nearly plane, and the inferior border is 

 transversely rounded behind the position of the incisive alveolus. The 

 The masseteric fossa is not defined below, and its anterior oblique bound- 

 ing ridge is indistinct. 



The alveolus of the third molar is close to the fundus of the incisive 

 alveolus, and is so shallow as to lead to the belief that it is readily and 

 early shed. That of the second molar is much deeper; it is small, and 

 its long axis is directed at 45"^ angle inwards and forwards. The ledge 

 between it and the base of the coronoid process is rather wide. The 

 masticating surface of the first molar is longer than wide, forming an 

 oval rather narrower anteriorly than posteriorly. Its extent on a line 

 drawn through the centers of the alveoli is twice as great as that of the 

 second molar. The fossettes are linear, and lie in three parallel lines. 

 The internal line contains three fossettes, the middle one three, and the 

 internal one two. The anterior one of the internal line does not extend 

 so far anteriorly as the extremities of the other two lines; its i)osterior 

 end also is not in line with the posterior fossette of the same line, but 

 begins opposite the space between it and the last fossette of the middle 



"^ See Americau Naturalist, July, 1G31, wliere this is douc. 



