1889.} ANATOMY OF POLYBOROIDES. 81 



the Falconidse. Polyboi-oides is in these particulars near to the 

 Falconidse. Cathnrtes is described and figured by Fiirbringer ^. 

 Serpentarius is neither figured nor described in Fiirbringer's work. 



The line of attachment of the deltoid to the humerus and, in 

 consequence, the size of that muscle vary in the three tvpes of 

 Accipitres. It is largest in Serpentarius and smallest in Cathartes ; 

 in the former bird the length of the attachment of the muscle to 

 the humerus is about half the entire length of the bone. In 

 Cathartes the proportion is (roughly) as 1:4. Gijpohierax is inter- 

 mediate but nearer to Serpentarius. Polyboroides agrees with 

 Gypohierax. 



In all Accipitres diurnse the anconeus has an accessory head 

 arising from the humerus close to the insertion of the latissimus dorsi. 

 But there are some differences of detail. 



In Serpentarius (fig. 2) the accessory head of the anconeus forms 

 a particularly broad flat tendon. The anterior of the two latissimi 

 dorsi end, as in most birds, in a narrow tendon ; this gives off a 

 branch to the integument just before its insertion. 



In Gypohierax the accessory head of the anconeus is very narrow, 

 but the relations of the latissimus dorsi to it and to the integument 

 are precisely as in Serpentarius. 



Cathartes is rather different from both these types ; the accessory 

 head of anconeus is almost completely split into two, the thickness 

 of the tendon being very unequal in different parts. The latissimus 

 dorsi tendon splits into two as in Serpentarius and Gypohierax ; 

 one tendon passes above and the other below the posterior latissimus 

 dorsi close to its insertion ; the uppermost of these is attached to 

 the belly of the anconeus. 



Polyboroides, as in other myological relations, comes nearest to 

 Gypohierax, but I am unable to state whether the branch of the 

 latissimus dorsi tendon to the integument is present. 



It is also worth remarking that while the scapular head of the 

 anconeus in Cathartes is distinctly double and entirely tendinous — 

 one tendon arising from the scapula itself, the other from the 

 supinator muscle — this muscle originates in all the remaining types 

 (including Polyboroides) from the scapula alone and by a single 

 head, which is chiefly fleshy though partly tendinous. 



The size of the second pectoral muscle offers characters by which 

 the affinities of Polyboroides can be to some extent determined. 

 In Cathartes aura the attachment of that muscle reaches nearly to 

 the end of the carina sterni. In Gypohierax angolensis the muscle 

 reaches only for a very short distance along the carina sterni ; this 

 is also the case with Serpentarius and Polyboroides. 



The proportions between the total length of the carina sterni 

 and the breadth of the second pectoral muscle where it is attached 

 close to the base of the carina sterni are indicated in the following 

 table : — 



' Loc. cit. pi. xxii. fig. 7. 



Proc. Zool. See— 1889, No. VI. 6 



