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PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
M- is somewhat smaller than M- 3 -, narrower behind and without the third 
posterior cusp which distinguishes this tooth in P. typicus. The palate 
is more concave in its posterior part, opposite P A , M 1 and M-, ascending 
rapidly between M-. Pg is considerably smaller than P3, but divided 
equally into two lobes by a perpendicular external groove as is not the case 
in P. typicus. The anterior lobe is notably narrower than the posterior 
and the inner side slightly grooved in its anterior part. The symphysis 
is very little shorter than in P. typicus , its posterior part extending 
beneath the anterior lobe of P T , but is much more horizontal. The 
mental foramina are two, both quite small. The first is a little forward 
of the anterior part of P^, and the second below the space between Pg and 
P^, considerably more forward than in P. typicus. Length of space occu- 
pied by the six upper molars .019 ; width of palate at P- .010 ; width of 
palate at M- .0135 ; length of space occupied by the six lower molars 
.019; width of I T .0037 I width of .002 ; length of symphysis inferiorly 
.0125; length of inferior diastema .006; depth of ramus below M T .011 
(’89, 429). 
Pachyrukhos trivius Ameghino. 
Pachyrucos trivius Amegh. ; Contrib. al Conoc., etc., p. 429, PI. 13, fig. 
31, 1889; Enum. Synoptique, etc., p. 19, 1894 (listed); Segundo 
Censo, etc., p. 151, 1898 (listed). 
A little smaller than the preceding species, although the dental series 
is of the same length, but the bones are more delicate, the mandible is 
lower and thinner and the molars, especially P^ of very different shape. 
This tooth is triangular and might be described as formed by a single 
lobe with anterior appendix, which ends forward in a thin vertical crest. 
This appendix is well demarcated from the lobe by a small external 
groove, but on the inner side is united with it into a single smooth plane. 
The mandible is distinguished by the almost uniform depth of the hori- 
zontal ramus below the teeth. On the most complete fragment of the 
mandible at my disposal there is no sign of the first mental foramen since 
the fragment is broken at this point. The second foramen, quite small, 
is below the anterior part of P T , somewhat farther back than in the pre- 
ceding species. The six lower molars occupy a space .018 in length; 
depth of jaw at M T .0095 (’89, 429). 
