OSTEOLOGY OF BIRDS 107 



as it is in Buteo, nor is the prebasal joint of the third or mid an- 

 terior toe. With regard to the fourth or outer toe, we find the 

 three proximal joints in the Falco all of about equal length; per- 

 haps the basal one being somewhat the shortest ; the next one rather 

 longer ; and the distal one of the three, the longest. This proportion 

 is just reversed in the Buteo, where the basal one is by far the 

 longest ; the next .being considerably shortened ; and the third very 

 markedly reduced in length. 



To all practical intents the pectoral and pelvic extremities of 

 other species of the genus Falco of this country agree in their 

 osteological characters with what I have just given for Falco 

 mexicanus. Most all of our hawks show an ossification of a 

 number of the tendons of the pelvic extremity, and many have the 

 "tibial cartilage" ossify to a greater or less degree. Falco 

 mexicanus, in adult individuals, exhibits two very sizable 

 ossifications in either tibial cartilage, which play the part of large 

 sesamoids at the back of the joint, articulating as they do with the 

 posterior aspect of the condyles of the tibiotarsus. 



Let up pass next to a consideration of the osteology of the cara- 

 caras. As I have already stated above, we have two species of these 

 birds in our avifauna, and both belong to the genus Polyborus — 

 Polyborus cheriway and Polyborus 1 u t o s u s . 

 But in the southern part of the American continent the caracaras 

 have a number of allied genera, as Ibycter, Doptrius, Milvago, 

 Phalcobaenus and Senex. Audubon's caracara or the Common 

 caracara (Polyborus cheriway) presents us with a good 

 example of a representative species. By nature, this bird is a 

 vulturine falcon, which subsists largely upon carrion, more or less 

 sluggish in flight and general behavior, terrestrial by habit, and 

 ambulatorial in gait. In view of these facts it will be interesting to 

 compare the skeleton of Polyborus with the characters as found in 

 the skeletons of the buteonine and falconine forms already con- 

 sidered. Hardly would we expect to find any cathartenine char- 

 acters present. 



The United States National Museum has placed at my disposal 

 some excellent material illustrating the osteology of Polyborus 

 cheriway, and I am especially indebted to my friend Mr Lucas 

 for the loan of a complete skeleton of Polyborus lutosus 

 which at the present writing is the only skeleton of the species in the 

 hands of science in this country. Also I have at hand skeletons or 

 parts of skeletons of Milvago, Doptrius, Ibycter and others more 

 or less nearly related. 



