454 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



? Sphceromys irruptus Amegh. ; Contrib. al Conoc. de los Mam. Fos. de 

 la Repub. Argent.; 1889, p. 169. 



This, the smallest known species of the genus, is distinguished not 

 only by its small size, but also by the character of m-, in which the pos- 

 terior prolongation is short, straight, directed backward, not obliquely 

 inward, and there is no groove demarcating the process from the second 

 lamina. The incisors are very narrow, hardly exceeding i mm. in width. 

 In the mandible p and m^ are smaller, m T and ^ somewhat larger. 



The upper grinding series measures 10 mm. in length, and the lower 

 9 mm. 



PROLAGOSTOMUS DIVISUS Ameghino. 



(Plate LXVIII, Fig. 16.) 



Prolagostomus divisus Amegh. ; Enumeracion sistem., etc.; 1887, p. 11. 



This species is considerably larger than the preceding one, from which 

 it farther differs in the larger size of the posterior prolongation of m-, which 

 is set off from the second lamina by a deep groove on the inner side ; the 

 size and direction of this process are subject to much individual variation 

 and may differ notably on the two sides of the same skull, but in all cases 

 it is larger than in P. pusillus and the demarcating groove is never absent. 



The incisors are narrow and slender, the thickness hardly exceeding 

 the width. In the lower series, p T is considerably and m^ slightly smaller 

 than m T and . 



The rostrum is strikingly long, narrow and tapering, but with relatively 

 large dorso-ventral diameter ; the palate is short and transversely concave, 

 with very prominent median ridge, and the posterior nares are large. 



In Ameghino's measurements of this and the following species are dis- 

 crepancies, probably typographical, which make it impossible to use them 

 for purposes of comparison. The two individuals in the Princeton col- 

 lection, from which the following measurements are taken, appear to be 

 somewhat larger than the type specimen. 



MEASUREMENTS. 



No. No. 



15,570 15,^82 



Upper dentition, length 036 



" pA-mi 014 



Upper incisor, width 002 



" " thickness 0023 



