OSTEOLOGY OF BIRDS 



269 



The articulation with the fibular ridge on the side of the tibiotarsal 

 shaft exceeds in length that portion of the bone that projects above 

 it, and equals in length the slender portion that is found below. The 

 connection between the bones along this ridge is of a ligamentous 

 nature, and the distal fibular end seems to be attached pretty much 

 in the same way to the side of the tibial shaft. This latter articu- 

 lation occurs at a point about the junction 

 of middle and lower thirds of the shaft of 

 the larger leg bone. 



With the exception of its proximal 

 fourth, the tarsomctatarsiis is considerably 

 compressed from side to side, much in the 

 same way as we find it in the Urinatoridae, 

 and to the same end. 



In order to show, comparatively, the 

 amount of this compression as compared 

 with a duck, for example, I have contrasted 

 this bone, in two views, with the same bone 

 taken from a specimen of the American 

 eider, a bird far less noted than the mer- 

 ganser as an habitual diver. This charac- 

 ter is here shown in figures 9-12. 



The hypotarsus of the bone in this mer- 

 ganser consists of four vertical ridges — 

 an inner large and longest one and three 

 others of equal length. They form the 

 grooves for the usual flexor tendons pass- 

 ing to the toes. In Lophodytes c u - 

 c u 1 1 a t u s the hypotarsus is twice 

 grooved, and to its inner side there is also 

 one small and complete perforation. 



Notwithstanding their lateral compression, the trochleae of the 

 distal end are very large, their median grooves distinct, and carried 

 all the way around. The inner trochlea is elevated upon the shaft, 

 and only descends as far as the base of the middle one. It is also 

 turned slightly inward, and at the same time projects the farthest 

 behind. The usual foraminal perforation is seen in the furrow 

 between the middle and outer trochleae, just above the cleft that 

 divides them [see fig. 9-12]. , 



We find the accessory metatarsal of a moderate size and elevated 

 far above the inner trochlear projection — not articulating with. the 



Fig. 9 Left tarsometatarsus; 

 anterior view Mergus ser- 

 rator [U. S. Nat. Mus. Osteo. 

 Collec. Birds, spec. 10626] 



Fig. 10 Same bone seen from 

 below 



Fig. 11 Corresponding bone, 

 same view, from Somateria 

 dresseri [U. S. Nat. Mus. 

 Osteo. Collec. Birds, spec. 

 16989] 



Fig. 12 Same bone as figure 

 11, seen from below. All figures 

 natural size and drawn by the 

 author from the specimens 



