234 NOTOUNGULATA order viii 



The Acoelodidae are distinguished from the Archaeopithecidae only by 

 their greater size, being about the size of a fox, and by the more oblique 



trapezoidal outline of the upper molars. 



Acoelodus, Oldfieldthomasia (Fig. 317), Eohyrax, 

 Acoelohyrax, etc., Ameghino. Eocene. Noto- 

 stylops beds of Patagonia. 



Family 6. Archaeohyracidae Ameghino. 



Skull similar to that of Hegetotherium, having, 

 however, weaker jugals and a more slender lower jaw. 



n,^«i^,i, !°'^^'\, ^ Dentition: '' ' ' • Superior incisors fairly large, 



Oldfieldthomasia sp. Upper Eocene 3 J 4.3 3 j ^ o 



(Notostylops bp.ds), Patagonia. A, ' ' ' ' . ti ^i i 



Upper cheek teeth. B, Lower second the loioer tucisors and canines peg-lifce, tiie lower pre- 

 and third molars. 1/1. ^^^^^^.^ moUriform. All of the upper teeth gradually 



inter grade from one form to the next ; thus the molars ha.ve an oblique, triangular 

 outline and have a parastyle like the premolars. All of the teeth are prismatic. 

 1/3 with large third lobe. 



These are probably the ancestors of Typotherium. Lower Miocene (Pyro- 

 therium beds) and Oligocene (Astraponotus beds). Archaeohyrax Ameghino. 



Suborder B. TOXODONTIA Owen.^ 



Extinct, digitigrade or semiplantigrade herbivores with three-toed extremities. 

 Dentition usually complete. Canines always weak, often lacking entirely. Molars 

 curved and lophodont. Clavicles wanting. Carpals alternating. Astragcdus toith rather 

 convex, slightly furrowed articulation for the tibia, distally foreshortened, articulating 

 only with the navicular. Calcaneum stout with a large articular surface for the 

 fibula. Humerus without eniepicondylar foramen. 



The Toxodontia are medium-sized and large herbivores, restricted to South 

 America, beginning in the Oligocene (?) of Patagonia and becoming extinct in 

 the Pleistocene of Argentina and Brazil. 



The skull is moderately high, its roof frequently being almost flat, and 

 sloping from the protruding rostrum to the occiput. The muzzle is slender 

 and rather long. The frontals are large, the parietals develop a weak sagittal 

 crest. The high and broad occiput is abruptly truncated downward, the 

 squamosomastoid region is not greatly inflated, and the exterior auditor}' 

 meatus opens obliquely upward. The condyles are rather prominent. The 

 zygomatic arch is unusually high but not very thick. Below the orbit, which 

 is open posteriorly, the arch curves downwardly and consists principally of 

 a process of the squamosal. The premaxillaries are elongated and high. The 

 lachr3nnal is small. The arched palate extends beyond the hindmost molars 

 so that the inner choanae are pushed far backward. The anterior palatal 

 vacuities are entirely surrounded by the premaxillaries. Lower jaw with 



^ BurmeLiter, H., Anales del Museo de Bueuos Aires, vol. i., 1867, and vol. iii. — Cope, E. D., 

 On To.vodon. Proc. Amer. Philos. Soc, 1881. — Lydekker, R., Anales del Museo de La Plata. 

 Paleontologia Argentina, 1890.— Owh, R., (Toxodon) in the Zoology of H.M.S. Beagle, 1840.— 

 Description of some species of Nesodou. Philos. Trans., 1853. — Roth, Santiago, Catalogo de los 

 maraiferos fosiles en el Museo de La Plata. Orden Toxodontia, 1898. 



