TO BIRD-ANATOMY AND CLASSIFICATION. 197 



The degree of development of this muscle (whose cut surfaces are marked 

 tf in fig. 5) varies much in different groups of birds, and in some may be 

 entirely absent, e. g. the Bucerotid and Palamedeidae. As a rule, it is 

 not small. In the paper on Chauna * a table will be found stating the 

 degree of development of this muscle in the greater number of families 

 (1. c. p. 199). 



(3) The semitendinosus. Generally a broad flat ribbon-shaped muscle, 

 bordering the contour (sometimes together with another muscle, the semi- 

 membranosus) of the fleshy part of the thigh behind. It arises from the trans- 

 verse process of the first free coccygeal vertebra, and from the fibrous mem- 

 brane between this and the ilium ; it is inserted into the inner side of the 

 head of the tibia (at least when the muscle next to be described is absent). 



(4) In many birds " a rhomboidal sheet of muscle, arising from the 

 anterior end of the linea aspera " (a muscular line on the under surface 

 of the femmr) joins the last-named muscle "anteriorly by an oblique 

 tendinous raphe, which continues down the back of the leg superficially." 

 When present, most of the main semitendinosus joins this accessory head 

 to be continued down the leg. 



(5) The femoro-caudal. This is a thin, narrow, elongated muscle, 

 which is covered above by the glutens primus (if present) and the biceps 

 cruris, below by the semitendinosus. Arising from the last two coccygeal 

 vertebrae, it is inserted into the linea aspera of the femur at about one 

 third its length (in the Fowl) from the trochanter. 



(6) In many birds the femoro-caudal is joined by an accessorius, which Ibis, 1881, 

 arises from the external surface of the pelvis behind the femur, and joins P- 12> 

 the main muscle, to be inserted with it into the femur, but nearer the head 



of that bone. 



As already mentioned, some birds possess all these six muscles, but no 

 bird is known which does not possess at least one. It is a convenient plan 

 to designate the four last-named muscles by the letters X, T, A, B. Thus 

 a Fowl possessing all four would have a formula A.B.X.Y, ihe> femoro- 

 caudal, accessory femoro-caudal, semitendinosus, and accessory semitendinosus 

 being all present. On the other hand, a Hawk or Owl, possessing only 

 one, the femoro-caudal, would have as formula A. 



Of these four muscles there are sixteen possible combinations ; but of 

 these only eight are actually found in birds t. 



Furthermore, if we denote the presence or absence of the ambiens 

 muscle by the signs + or , we obviously get sixteen combinations 

 again, though of these only fifteen have, as yet, been observed. For the 

 sake of brevity it will be useful to remember the above four letters, to 

 save the repetition of the full names. To the bearing of these muscles 

 in the classification of birds, I intend to revert at a subsequent part of 



* " On the Anatomy of Chauna derbiana," &c., P. Z. S. 1876, pp 189-200. 

 t P.Z.S. 1874, p. 111. 



