Individual Variation.—The anterior edges of the two adductor muscles are strongly fused together in some cases.
T. cupido
Individual Variation.—The anterior edge may be fused with that of M. adductor superficialis. The distal end is sometimes slightly fused with M. gastrocnemius pars media. In one leg, the proximal two thirds of the insertion is entirely tendinous, whereas in another the distal end of the insertion is tendinous.
P. p. jamesi
Individual Variation.—The anterior edge (in one leg the entire muscle) in some legs fuses with that of M. adductor superficialis.
M. Obturator (M. obturator externus + M. obturator internus), [Figs. 16], [17], [18], [19]C, D, [20]K, L, M
I am adopting the single name M. obturator for the complex that Fisher (Fisher, 1946; Fisher and Goodman, 1955) subdivides into Mm. obturator externus and obturator internus. The reasons for this change are given in the section on terminology.
For ease of description, it is desirable to apply names to the subdivisions of M. obturator. It has been customary to divide the obturator complex into two parts—an obturator internus and an obturator externus; the latter has often been further subdivided. The evidence given below demonstrates that a primary division of the complex into only two parts is unsatisfactory.
I strongly suspect that comparable parts of the obturator complex have been considered a part of the "internus" in some birds and a part of the "externus" in others. In their work on the Galliformes, Hudson, et al. (1959) subdivide the obturator complex into only two divisions—obturator externus and obturator internus. The extrapelvic part of this complex that arises from the rim of the obturator foramen and inserts in common with the stout tendon of the main intrapelvic part of the obturator internus is considered by them to be a part of the obturator internus. Their obturator externus lies anterior and deep to the extrapelvic part of the obturator internus and inserts separately from the latter. (I also have found this same arrangement in Tympanuchus and Pedioecetes.)
Berger (1952), in his description of the Black-billed Cuckoo (Coccyzus erythrophthalmus), also divides the obturator complex into an obturator internus and an obturator externus; the latter he subdivides into a dorsal and a ventral part. He states (p. 530) that he did not find any measurable differences in myology between C. erythrophthalmus and C. americanus. In order better to compare this arrangement with that in Tympanuchus, I have examined two specimens of C. americanus. My findings in the latter differ from Berger's description (p. 541) in one respect. Whereas Berger states that the dorsal and ventral parts of M. obturator externus are distinct except at their origin, I find them fused for their entire length; the muscle fibers that connect these two parts lie deep to the tendon of M. obturator internus. The origin of all parts of the complex in Coccyzus is similar to that in Tympanuchus. The only notable difference in configuration is that the part in Coccyzus that appears to correspond to the obturator externus of Hudson, et al. (1959) is not separate from the remainder of the extrapelvic part of the muscle. Berger (1952) considers all parts of the muscle having an extrapelvic origin to make up the obturator externus. It appears to me that the dorsal part and a part of the ventral part of the obturator externus of Berger correspond to the extrapelvic fleshy part of the obturator internus of Hudson, et al.