OSTEOLOGY OF BIRDS 95 



ing a true posteroexternal angle. (Compare with figure of Circus 

 where these edges are uniformly rounded.) The.vonier usually has 

 its apex resting against the rear point of union of the maxillopala- 

 tines, and these latter are composed of a very open tissue. Basi- 

 sphenoidal processes of a very rudimentary character are present 

 just as we find them in Circus. In the eyeball the sclcrotal plates 

 are very large. 



There are but three characters by which the remainder of the trunk 

 skeleton in Buteo 1 i n e a t u s can with certainty be distinguished 

 from that of Circus hudsonius, apart from the very slight 

 difference in size. Buteo 1 i n e a t u s may have as many as 

 eight costal ribs articulating with either sternal border; in the pelvis 

 the ilia are in contact, or very nearly in contact, across the sacral 



Fig. 47 Right lateral view of the pelvis of Buteo borealis calurus, show- 

 ing the free hinder portion of the postpubic element (p' ) ; in, the interval which occurs 

 between it and the obturator foramen (op') Life size from the specimen 



crista; and the decussation of the coracoids in their sternal grooves 

 is very small, and even may not exist. To these slender differential 

 characters I would add the fact that the postpubic spine upon either 

 side of the pelvis in the Buteo usually closes in the obturator 

 foramen. Specimen no. 6643 of the collections of the United 

 States National Museum, marked Buteo pennsylvanicus 

 (Buteo 1 a t i s s i m u s ?) has the sternum without either 

 notches or fenestra, but this also sometimes is seen in Circus. 



Passing to the skeleton of the pectoral limb the differences are 

 barely worthy of mention between Circus and Buteo I i n e a t u s , 

 and the same remark applies with about equal truth to the skeleton 

 of the pelvic limbs in these two species. Relatively the tarsomet- 

 atarsus is somewhat shorter in Buteo 1 i n e a t u s , and the fibula 

 is longer in the Buteos. 



