MEGATHERTID^. 105 



The forms ranged under Pseudolestoclon connect the typical species 

 so closely with those of the so-called Lestodon that it seems advisable 

 to include the whole of them under one generic title ; an analogous 

 remark applies to the so-called Grypotherium^ . 



Teeth -J ; those of the upper jaw are usually suhtriangular or oval 

 in section, the hinder ones being grooved internally ; the first three 

 lower teeth are very similar in structure, but the fourth is elongated 

 and has an inner crochet. In the typical forms there is only a small 

 interval between the first and second teeth, and the plane of wear 

 of the first tooth in both jaws is horizontal ; in other species, how- 

 ever, the diastema is longer and the plane of wear of the first tooth 

 oblique, as in Cholcepus. The mandible has no inferior descending 

 plate. The humerus has no entepicondylar foramen. The tibia 

 is relatively short, and has a notch on the anterior aspect of the 

 distal extremity for the reception of the astragalus. The astragalus, 

 which varies considerably in the different species, approaches that of 

 Megatherium, but has no navicular cup. The terminal phalangeals 

 of both limbs are narrow and straight, and there is no anchylosis of 

 any of the phalangeals ; the first, second, and third digits of the 

 manus, and the second and third of the pes are furnished with claws ; 

 the third digit of the manus being the longest. The scapula closely 

 resembles that of Scelidotherium. There are numerous small ossifi- 

 cations in the dermis. 



The genus approaches the nearest of any member of the family to 

 the Bradypodidce, species like M. gracilis and M. armatus being 

 those in which this approximation is the most marked. 



Group A. 



The premaxillae elongated and united anteriorly by a bony arch 

 with the nasals ; the anterior portion of the mandible equally elon- 

 gated, and longer than the dental series. 



Mylodon darwini^ Owen^ 



Syn. Grypotherium darwini, Reinbardt^. 

 Scelidotherium ankilosopum, Bravard *. 



This is the only species of the present group, and is nearly equal 

 in size to M. armatus. The length of the nasals is 0,126, that of 



^ See Burmeister, Descript. Phys. Eep. Argentine, vol. iii. pt. 1, p. 358 

 (1879). 



* Zoology of the Voyage of the ' Beagle', pt. 1, p. {)8 (1840). 

 3 K. Danske Yid. Selsks. Skr. ser. 5, vol, xii. p. 353 (1879). 



* In P. Gervais, Zool. et Pal. Gen6i-ales, s6r. 1, p. 132 (1867-69). 



