186 PROCEEDINGS OP THE ACADEMY OP 



ity of the bone is still more peculiar showing a characteristic of 

 the family only shared, to a certain extent, by Alca impennis. (Fig. 

 6). The bone is cut off obliquely, at an angle of about 45, with 

 a straight edge. The anterior three-fifths of this is occupied with 

 the two facets for articulation with the bones of the forearm. 

 These facets are continuous, and the radial one scarcely shows 

 the particular elliptical shape common among birds. The pos- 

 terior extremity of this oblique end of the humerus is prolonged 

 far beyond the true elbow joint, furnishing a resting place for the 

 two large anconal sesamoids. The very end is deeply grooved, 

 and another similar groove is impressed upon the bone on the 

 outside, a little higher up. The sesamoids are in these grooves. 

 The structure of the elbow is such as to allow but very restricted 

 motion ; the forearm can be flexed out slightly from its usual posi- 

 tion of nearly complete extension. 



The larger, outer one of the two sesamoids is an irregularly 

 discoidal bone, with a smooth, flat, external face, and curved outer 

 border; its sharpened edge fits in the upper of the two grooves 

 at the extremity of the humerus; the prominence of the hu- 

 merus that defines this groove abuts against a slight excavation on 

 the outer surface of the sesamoids. The smaller sesamoid lies a 

 little behind and inside the other; it has one flat face applied 

 against the surface of the other, an irregular surface with a cen- 

 tral ridge fitting into the extreme groove of the humerus, and a 

 long, curved, postero-external free border. These sesamoids, 

 borne upon the most projecting point of the humerus, afford an 

 extremely powerful leverage for extension of the forearm. 



The ulna and radius are similar to each other in general appear- 

 ance, both being laminar, like the humerus, with a sudden thick- 

 ening and constriction at the proximal extremity, so that the 

 articular facets are irregularly circular. The distal extremity of 

 the ulna is pretty effectually divided into two facets, a posterior 

 excavation in which the head of the ulnare articulates, and a 

 superior prominence abutting against a surface of the radiale. The 

 distal extremity of the radius shows a single convex articular sur- 

 face fitting the concavity of the radiale. The outer surface is 

 obliquely impressed for a long distance with a distinct groove for 

 a tendon ; the anterior margin of the radius is a straight, sharp 

 crest, more prominent than the opposite edge of the ulna. The 

 two bones of the forearm have verv intimate relations to each 



