ON THE ANATOMY OF CHAUNA DERBIANA. 325 
flexor surface of the humerus. In certain birds this muscle sends off 
from-its upper end a slender fusiform belly, which runs through the 
proximal portion of the patagium to join its marginal tendon near the 
middle of its course (Plate [16] XIV. fig. 2). The presence or absence 
of this muscular fasciculus is a very constant character among closely 
allied birds. In the Table (p. 329) are recorded the names of all 
those birds in which, according to my experience, it is to be found. 
The only Anomalogonatous birds in which I have seen it are the 
pereealeide. 
-3. The Area of Origin of the Obturator internus. 
It is not my-intention on the present occasion to enter into the 
consideration of whether the muscle here called obturator internus is 
homologous with the same-named muscle in Mammalia; suffice it to 
say that it arises from the pelvic surface of the pubis and ischium, 
and ends by a tendon which is inserted into the outer surface of the 
head of the femur. 
In a large number of birds, on looking at the pelvic view of this 
muscle when undisturbed, its shape is seen to be an elongated oval, 
occupying the obturator fossa, and covering the line of junction of the 
_ischium and pubis. In another large number of birds, instead of 
being oval it is triangular, its posterior fibres expanding in such a way 
as to cover most of the pelvic surface of the ischium. There are a 
few birds in which an intermediate condition is found; they are, how- 
ever, very few. In most there is not the least difficulty in deciding 
whether the obturator internus is oval or triangular (compare 
Plate [17] XV. figs. 1 and 2). From the Table (P. 329) the arrange- 
ment existing in most birds can be found. 
4. The degree of Development of the Tensor-cruris fascie. 
To this point I have referred in my paper on the muscles of 
Birds,* where its relations are explained. “It is the superficial 
muscle of the outside of the thigh, covering the femur. It is flat and 
triangular in shape, and arises as a membranous expansion which 
covers the gluteus ii., from the lower two-thirds of the posterior 
border of the iliac fossa in which that muscle is situated, and from 
the fibrous septum which separates that muscle from the gluteus iii. 
Further down it has origin also from the whole length of the ridge 
which separates the postacetabular area from the external lateral 
surface of the ischium, and which may be termed the postacetabular 
ridge, as well as from the posterior border of the ischium, as far 
* “Proceedings of the Zoological Society,” 1873, p. 628. (Supra, p. 189.) 
