1889.} ANATOMY OF POLYBOROIDES. 8h 
the Falconide. Polyboroides is in these particulars near to the 
Falconide. Cathartes is described and figured by Fiirbringer’. 
Serpentarius is neither figured nor described in Firbringer’s work. 
The line of attachment of the dedéoid to the humerus and, in 
consequence, the size of that muscle vary in the three types 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. Gypohieraz is inter- 
mediate but nearer to Serpentarius. Polyboroides agrees with 
Gypohierax. 
In all Accipitres diurnze the anconeus has an accessory head 
arising from the humerus close to the insertion of the datissimus dorst. 
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 datissimi 
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 /atisstmus 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 datissimus 
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 Iam unable to state whether the branch of the 
datissimus 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 Gypohierar 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 
en to the base of the carina sterni are indicated in the following 
table :— 
* Loe, cit. pl. xxii. fig. 7, 
Proc. Zoo. Soc.—1889, No. VI. 6 
