SUBORDER E LITOPTERNA 123 



distal artiadar surface. Cakaneum articiUating with fibula. Dentition complete or 

 ivith incisors reduced. Teeth often in close series. Cheek teeth bunolophodont, seleno- 

 lophodont, usually brachyodont. Humerus without entepicondylar foramen. Terminal 

 phalanges broad. 



The Litopternine suborder of ungulates is restricted to South America. 

 It developed from a North American bunodont condylarth ancestor, and in 

 so doing acquired a perissodactyl-like dentition and reduction of the outer 

 digits of the feet such as occurs in many perissodactyls. 



The reduction sometimes involves only the first and fifth digits, but the 

 second and fourth may also be reduced, either becoming shorter and thinner, 

 or becoming mere rudiments. The teeth become only exceptionally hypso- 

 dont, and those which remain brachyodont become more highly specialised 

 in that the roots are further divided, so that each tooth in the lower jaw is 

 provided with four roots. They have, in common with the Notoungulata, a 

 short metaloph on the upper molars, but are distinguished from these, not 

 only because they are brachyodont, but also because the trigonid and the 

 talonid of the lower molars are about the same size. The upper molars have 

 two outer cusps, which form an ectoloph made up of para- and metastyle, 

 a large protocone, two secondary cusps and a small hypocone. The protocone 

 is joined with the protoconule to form an oblique protoloph, the short 

 metaloph is made up of the hypocone and the rather prominent back surface 

 of the tooth. The metaconule is connected rather with the protocone than 

 with the hypocone. The lower molars consist of two semicrescental ridges, 

 of uniform size. The inner cusp arising to one side of the second ridge, a 

 feature which is very characteristic of the Notoungulata, is found only in the 

 Macraucheniidae. The last of the incisors may become vestigial, and the 

 second larger than the first. All of the teeth are, as a rule, brachyodont and 

 are set rather closely. The bones of the skeleton are very similar to those 

 of the perissodactyls. 



Family 1. Bunolitopternidae. {Didolodidae Scott.) 



Upper molars with two outer cusps, which are more or less separated from the 

 rest, two large inner cusps, of unequal size, and frequently with a nurnber of secondary 

 cusps. Lower molars with tivo conical inner cusps and two \/-shaped outer cusps. 

 Premolars much simpler than the molars. 



This family is distinguished from the condylarths principally by the 

 small canines and the lack of a diastema. The exti'emities were possibly 

 pentadactyl. 



Among the numerous genera which Ameghino established, basing his 

 classification on isolated molars, Lambdaconus, Oroacrodon, etc., seem to lead 

 to the Macraucheniidae ; Notoprogonia, Lonchoconus, Proectocion, etc., to the 

 Proterotheriidae ; and others, such as Pacardolydekheria, Josepholeidya, Argyro- 



Rev. Jard. Zoolog. Bueuos Aires, 1894. — Enumeration synoptiqne des nianimiferes fossiles eocenes 

 de Patagonie. Buenos Aires, 1894. — Mammiferes cretaces de 1' Argentine. Bol. Instit. Geograf. 

 Argent. Buenos Aires, 1897, vol. xviii. — Recherches de morphologie sur les molaires superieures 

 des ongules. Anal. Mus. Nac. Buenos Aires, vol. ix. , 1904. — Burmeister, Herm. Anal. Mus. 

 Publ. Buenos Aires, 1864, vol. i. — Nova Acta Acad. Leop. Carol., 1885, vol. xlvii. — Cope, E. D., 

 The Litopterna. Amer. Naturalist, 1889, vol. xxv. — Gaudry, A., Fossiles de Patagonie. Mem. 

 Soc. Geol. France. Paleont. vol. xiii. 1904. — Annal. dePaleont., 1906. — Lydekker, R.^V&lA&onto- 

 logia Argentina. Anal. Mus. de La Plata, 1893. — Scott, W. B., Litopterna of the Santa Cruz Beds. 

 Rep. Princeton Exped. Patagonia. Princeton, 1910. 



