362 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
PROTHYLACYNUS Ameghino. 
(Plates XLVII-LI ; LII, Figs. 3, 5 ; LIII, Figs. 5-8 ; LIV, Figs. 2, 2 a, 8, 9, 14 ; LXI, Fig. 2.) 
Prothylacynus Amegh.; Nuevos Restos. Mamif. Fos. Patagonia Austral, 
p. 26, Aug. 1891 ; Revista Argentina Hist. Nat. I, entr. 5 a, p. 312, 
Oct., 1891. 
This genus, so frequently mentioned in previously published discussions 
of the relationships of South American and Australian marsupials, is 
represented in the Princeton collection by the remains of one individual 
referable to the species Prothylacynus patagonicus (No. 15,700), comprising 
an incomplete skull, mandible, parts of both scapulae, humerus, both radii, 
ulna, pelvis, both femora, tibia, fibula, patella, part of the hind foot, the 
scaphoid and a few phalanges of the fore foot, six cervicals, four dorsals, 
three lumbars, the sacrum, five caudals and fragments of the ribs and 
sternum. The exceptionally perfect state of preservation of this material 
permits full comparison with living forms and indicates in no uncertain 
fashion the true relationships of the animal, a discussion of which will be 
found on a later page. 
Dentition (Pis. XLVII ; XLVIII, figs. 1, 2). — The greater part of the 
facial region of the skull has been weathered away, involving the incisors, 
canines and the anterior and median premolars. In the side view (PL 
XLVII), these have been supplied from Borhycena. Prothylacynus dif- 
fers, however, in possessing an additional upper incisor (Ameghino, 1894, 
p. 12 1). The posterior superior premolar is a stout, double-rooted tooth, 
with a broad heel without heel cusp. It is considerably smaller than this 
tooth in Thylacynus . The first, second and third molars increase rapidly 
in size posteriorly. In M- and M~, the protocone is a well-developed 
bunoid cusp, but is absent in M~, although the inner root supporting this 
part of the tooth crown is larger than in the preceding molars. The 
antero-external style, the only one of the outer row of styloid cusps 
remaining, is considerably larger than in Thylacynus. With this excep- 
tion, the outer cingulum is entirely wanting. The fourth molar is more 
reduced than in Thylacynus. The protocone is represented by a broad 
smooth surface. The metacone is vestigial and the posterior root sup- 
porting it almost obliterated. The high conical paracone is connected by 
a sharp ridge with the antero-external style. 
In the mandible, the crowns of the three incisors have been broken off, 
