396 ON THE PETRELS COLLECTED DURING 



The semi-tendinosus (t. c., figs. 1-3, s.t.) muscle is always present and 

 strong. It arises from the iliac bone round its most posterior angle, and 

 has no connection at all with the caudal vertebrae. Anteriorly it slightly 

 overlaps the biceps. 



Excepting in the Oceanitidae, it has no accessory head, so that all 

 its fibres are inserted by a thin, broad tendon, quite distinct from that of 

 the semi-membranosus, on to the tibia. In the Oceanitidse, in all the 

 species and genera, there is a strong and broad accessory head, arising 

 from the femur, as usual, and joining the main belly of the muscle by an 

 oblique tendinous raphe (vide PI. XX. fig. 3, a.s.t.}. 



The semi-membranosus (t. c., fig. 1-3, s.m.) is always very large, of 

 broad, flat, ribbon shape, not so parallel with the semi-tcndinosus nor so 

 covered by it, as usual, its direction being more oblique than is that of 

 the other muscle, and thus more parallel to that of the adductors. It 

 arises from the posterior margin of the ilium, ischium, and pubis, from a 

 little above the ischial prominence to within a small distance of the end 

 of the pubis. Its insertion is by a thin, broad tendon, anterior to, and 

 separate from, that of the semi-tendinosus. 



Of the other muscles in the hind limb the biceps always passes through 

 a tendinous loop on its way to its insertion, as is nearly always the case 

 with birds. 



The obturator eocternus is never large, and is inserted near the femur 

 head. The obturator internus is of peculiar shape, nearly oval, but with 

 a slight indication of becoming triangular. The deep flexors of the toes 

 and of the hallux blend, usually about halfway down the leg, and their 

 tendons may become ossified. Even when a hallux is present it receives 

 no tendon at all from these muscles. 



4. Other Anatomical Features. 



There are always two carotid arteries situated in the hypapophysial 

 canal. There are also two jugular veins, the right of which is frequently 

 the largest. 



The main artery of the leg is always the sciatic one, accompanying as 

 it does the sciatic nerve. 



In the genus Pelecanoides, as has already been described by Grarrod 

 (c/. antea, p. 373), the femoral vein, instead of being, as usual in birds, 

 deep of the femoro-caudal muscle, from the external border of which 

 it then seems, in the ordinary course of dissection for the thigh-muscles, 

 to emerge (PL XX. fig. 2, /.v.), is superficial to it, appearing at the 

 external edge of the obturator externus, and then crossing the femoro- 

 caudal superficially as represented in PI. XX. fig. 4, f.v. 



Zool Chall. I n the Procellariidse, except Pelecano'ides, the two most anterior air- 



Exp. vol. iv. C ella, which lie between the rami of the furcula at the entrance to the 



thorax, are not, as is usually the case in birds, fused together to form an 



