117 



face (h) ; this is of less extent than the one above ; its plane is vertical, looking 

 directly inwards, and is adapted to the flat surface on the outer side of the astra- 

 galus ; it is situated towards the anterior part of the inner surface of the malleo- 

 lus ; the posterior rough tract is of nearly equal extent. 



Bones of the hind-foot*. — The terminal segment of the hinder extremity is re- 

 markable both for its strength and especially its length, measuring not less than 

 sixteen inches in the latter dimension : the tarsal portion is a model of massive 

 organic masonry : the toes maintain a close analogy with those of the fore-foot, 

 and, like them, are modified, some for the envelopment of a hoof, others for the 

 support of long and powerful claws. 



Of the tarsus there have been preserved the astragalus, calcaneum, cuboides, 

 naviculare, the external cuneiform and a smaller cuneiform bone ; which six 

 bones, it will appear by the following descriptions, are all that the Mylodon 

 possessed in this segment of the foot. 



The astragalus f is of an irregular pyramidal form, the posterior end forming 



the apex, and the base, which is turned forwards, being rudely divided into three 



large tuberosities. If the foot be placed with the sole flat on the ground, as in 



Plate XXL, the astragalus, naturally coadapted to the other tarsal bones, has its 



fibular or outer side uppermost, and a great part of the articular surface for the 



tibia looks inwards ; when articulated, therefore, to the leg, placed vertically above 



it, the foot rests upon the ground by its outer edge, not by its sole, and the 



peculiarities of the metatarsal structure relate to this inversion of the foot. The 



articular surface which the astragalus presents to the bones of the leg is divided 



into three parts, the general planes of which are at right angles to each other. The 



middle division of the surface|, which, in the naturally inverted position of the foot, 



is horizontal, presents a reniform figure ; it is shghtly convex anteriorly, concave in 



a less degree posteriorly : at the antero-lateral part of its outer convex border the 



articulation adapted to the malleolar process of the fibula is continued upon the 



external surface of the astragalus, presenting anteriorly the form of a semi-oval 



slightly convex smooth tract, and contracting suddenly behind this part to the 



breadth of two or three lines. The third or internal division § of the upper 



articular surface of the astragalus, which in ordinary cases receives the internal 



• Plates XXI. and XXII. t Ibid., a, and PI. XXIII. figs. 1 and 2. 



J PI. XXIII. fig. 1. a. ^ PI. XXIII. fig. 1. b. 



