346 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



shallower; the coronoid process is broader and less recurved and does not 

 rise so high above the condyle ; the angular process is shorter, but much 

 broader dorso-ventrally, and thus the whole hinder portion of the jaw has 

 a characteristic appearance. The postero-external opening of the dental 

 canal is on the horizontal ramus below the base of the coronoid, and 

 there is a single large and conspicuous mental foramen on each side of 

 the predental beak, near the dorsal border. 



The few known vertebrae differ but little from those of Hapalops. The 

 limb-bones are short and heavy and the humerus, at least, has a decided 

 resemblance to that of Mylodon, as is also true of the first metacarpal. 

 The astragalus is characteristically different from that of either of the 

 preceding families ; it has much the same narrow, elongate shape as in 

 the Megalonychidce, but the inner condyle has already begun to assume 

 the odontoid form and suggests that of Mylodon. 



On account of the rarity and incompleteness of the material, the tax- 

 onomic subdivision of this family is an even more difficult task than in the 

 case of the Megalonychidce, and the difficulty is increased by the great 

 variability in the forms of the teeth. Indeed no satisfactory arrangement 

 is yet possible and the scheme here adopted is merely provisional. 

 Ameghino has named four genera and nine species, but this seems to be 

 an unnecessarily large number, and is almost equivalent to making a 

 different species for every individual. 



NEMATHERIUM Ameghino. 



(Plates LXII ; LXIII, Figs. 2-8.) 



Nematherium Amegh. ; Enum. Sistem. etc., 1887, p. 22. 



Ammotherium Amegh.; Rev. Argent, de Hist. Nat, T. I, 1891, p. 321. 



Lymodon Amegh. ; Ibid., p. 324. 



The principal characteristic of this genus is the decided anterior diver- 

 gence of the two dental series, which is displayed in both upper and 

 lower jaws, and the consequent broadening of the palate forward, though 

 it never approximates the condition seen in Mylodon. Within certain 

 limits, the teeth vary much in shape ; - is usually cylindrical, though it 

 may be triangular, and, in most instances, is bevelled by abrasion, but 

 may be horizontally worn ; the other upper teeth are triangular, varying 

 much in proportions and in mode of wear ; -, * and - are usually bevelled, 



