458 PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



PLIOLAGOSTOMUS Ameghino. 



Pliolagostomus Amegh. ; Enumeracion sistem., etc.; 1887, p. 12. 



This very rare and imperfectly known genus is closely allied to the pre- 

 ceding one, but differs from it: (i) in the shortness of the lower incisor, 

 which reaches only to p T , instead of extending the whole length of the 

 grinding series, and therefore the protuberance along the inner side of the 

 mandible, so conspicuous in Viscaccia and Prolagostomus, is wanting; 

 (2) in the form of the grinding teeth, which are much less oblique to the 

 axis of the jaw and which, at least m^ and , have as great an antero-pos- 

 terior diameter internally as externally ; the same teeth have a definite 

 inner side, which forms an angle with the anterior side, not curving into 

 it gradually. Furthermore, in some of the teeth, notably m T , there is an 

 internal groove between the laminae, not so deep as the outer groove, but 

 quite distinct. 



In the Ameghino collection are some upper molars referred to this 

 genus, which differ from those of the mandible in being of more oval 

 shape, while the posterior prolongation of m-, or incipient third lamina, 

 is directed transversely, not backward, or obliquely inward, as it is in the 

 preceding genus. 



The mandible resembles that of Prolagostomus in shape and in the 

 small size of the masseteric crest and fossa. 



Too little is known of Pliolagostomus for any definite determination of 

 its phylogenetic significance, but it appears to be merely an aberrant 

 member of the family, which has left no descendants. 



PLIOLAGOSTOMUS NOTATUS Ameghino. 



(Plate LXVIII, Fig. 20.) 



Pliolagostomus notatus Amegh. ; Enumeracion sistem., etc.; 1887, p. 12. 

 This is a very small animal, exceeding Prolagostomus pusillus but little 

 in size. In the type specimen p- 4 - and r% are considerably smaller than 

 m T and ^. In the Princeton collection is a mandible (No. 15,087) of a 

 very young animal, in which p T , though erupted, had not yet come into 

 use, which differs from the type in several respects and, though of similar 

 size, may possibly represent a different species ; p^ is very small, but rr% 

 is larger than m T , and the mandible is deeper anteriorly. 



