312 PATAGONIAN EXPEDITIONS! PALEONTOLOGY. 



two columns, of which the external is much larger and is placed in the 

 median line of the tooth, with a broad shallow groove on its outer face ; 

 the internal column is very much smaller and is sharply defined by deep 

 vertical grooves in front of and behind it ; ^ is of the ordinary transversely 

 oval shape, but has a distinct, vertical sulcus on the inner side ; T is sub- 

 cylindrical and quite normal. 



PELECYODON PETR^EUS Ameghino. 

 Pelecyodon petrceus Amegh. ; Rev. Argent, de Hist. Nat, T. I, 1891, 



P- 323- 

 This species is as yet represented only by the type specimen and, 



although distinguished merely by the form of the teeth (usually a very 

 uncertain criterion) is probably valid. Its peculiarities are not connected 

 by transitional forms with the other species, but, on the contrary, give it 

 an isolated position. On the left side, A is shaped much as in P. cristatus, 

 elongate antero-posteriorly and much compressed laterally, while on the 

 right side it is relatively broader ; * is narrower anteriorly than in the pre- 

 ceding species and thus has a more trihedral form ; ^ is remarkable for its 

 shortness antero-posteriorly and transverse width and is in marked con- 

 trast to the shape of this tooth in the other species of the genus ; - is 

 smaller, but of similar, exceptionally short and broad form ; - is very small 

 but relatively very wide. 



The following dimensions are from a photograph of the type, right side. 



MEASUREMENTS. 



Upper dentition, length 032 %-, length 0045 



" -- 5 023 " width 0095 



J-, length (t. f., antero-posterior diam- A, length 0035 



eter) 0065 " width 009 



" width (t. f., transverse diameter) . . . .0035 &, length 002 



3 , length 0x565 " width 006 



" width 006 



PELECYODON ARCUATUS Ameghino. 



(Plate LIII, Figs. 2-7.) 

 Pelecyodon arcuatus Amegh.; Rev. Argent, de Hist. Nat, T. I, 1891, 



P- 323- 



In the collection of the American Museum of Natural History is a 

 specimen (No. 9,240) which agrees closely with the type of this species. 



