Vol. 55.] OF A MiMMALTA-N HUMERUS FROM TON-BRIDaB. 415 



depression. Above the condyles in front there is the usual supra- 

 condylar depression, somewhat deep and narrow. 



On the posterior aspect the distal end shows its most distinctive 

 features. A long narrow olecranon-pit extends above the small 

 external condylar surface. It is ovate in outline, filled with limonite, 

 is 1| inch long and fully | inch wide in the middle. The inner 

 border is comparatively straight and vertical, and the outer border 

 is convex. 



The small external condylar surface below the olecranon-pit is an 

 inconspicuous convexity about | inch deep. No part of the inner 

 condyle is visible on the posterior aspect of the bone. 



The bone which margins the upper half of the olecranon-pit on 

 the inner side is y^^- inch wide, rounded from side to side, but slightly 

 worn. The external margin of the pit, which may also be a little 

 worn, is about half as wide. This character of the relatively external 

 position of the olecranon-pit is seen in the humerus of the tapir, 

 and to some extent among horse's. 



The lower half of the process which usually borders the olecranon- 

 pit is absent in this fossil ; and on the inner side, in its place, is an 

 ovate cartilaginous surface, concave in length, inclined obliquely 

 inward, making an angle of about 45° with the axis of the shaft. 

 There is a similar, but much smaller, atrophy of the corresponding 

 process on the external side, giving rise to a truncated surface, 4 

 inch deep, forming a narrow crescentic impressed area. This 

 character defines a lateral concavity between the hinder end of the 

 humerus and the anterior border of the olecranon-process of the 

 ulna. This truncation of these parts of the humerus is especially 

 common in antelopes and deer ; but, in comparison with known 

 types, recent and fossil, it is much more developed in the Tonbridge 

 fossil animal. There is no Tertiary mammal in which the character 

 is so conspicuous as in existing types. If Hyracotherium or Plio- 

 lophus has the distal end of the bone placed as far forward on the 

 shaft, it entirely wants the truncation of the bone on the lower 

 borders of the olecranon-pit ; and there is no other fossil genus 

 with which the specimen has a closer affinity. This consideration 

 is perhaps evidence against the fossil being derived from some 

 Tertiary stratum and accidentally left where it was found, and so 

 far is favourable to the specimen being of Wealden age. 



On the whole, the weight of evidence from comparison with other 

 types appears to incline towards reference of the fossil to the Artio- 

 dactyla, though there are almost as many points in common with 

 the Perissodactylate humerus. In neither is the hinder part of the 

 distal articulation of the bone comparable to this fossil. I therefore 

 infer that it indicates a new family type. The teeth described by 

 Mr. Smith Woodward and by Mr. Lydekker are indicative of much 

 smaller mammals from the Wealden Beds, and the interest of the 

 specimen now described is chiefly in its size. It may be known as 

 Hemiomus major, gen. et sp. nov., in reference to the absence of 

 ossification of the hinder aspect of the distal end of the bone, and' 

 as indicative of its size. 



[For the Discussion on this paper, see p. 418.] 



