REPORT ON THE ANATOMY OF THE PETRELS. 31 



absent. It is fairly broad and ribband-shaped, overlapping the semi-membranosus in the 

 Oceanitidse at its origin, and inserted into the femur together with the femoro- caudal. 

 In the genera Fregetta, Puffinus, and Majaqueus it is decidedly small. 



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 Oceanitidse, 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 PL V. fig. 3, a.s.t.). 



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

 ribband shape, not so parallel with the semi-tendinosus 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 externus 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 

 half way 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 Garrod (cf. antea, p. 10), 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 V. fig. 2,f:v.), — is superficial to it, appearing at the external 

 edge of the obturator externus, and then crossing the femoro-caudal superficially as 

 represented in PL V. fig. i,/.v. 



