REPORT ON THE ANATOMY OF THE PETRELS. 29 



In nearly all the other Procellariidse, including Pelecanoides, the biceps becomes 

 modified in a peculiarly interesting way. The coracoid head alone forms the muscle 

 proper, whilst the humeral head, becoming detached from the coracoid head, goes entirely 

 to the tensor patagii longus tendon, which it joins as a short, cylindrical tendon close to 

 the shoulder (PI. III. fig. i, h.). It is, therefore, functionally a " biceps-slip," though it 

 differs from the ordinary " biceps-slip " found in so many birds, 1 in that it arises inde- 

 pendently from the humerus, and is not a part of the true biceps muscle, although it is 

 supplied by the same nerve as that which goes to the coracoid head. In Diomedea, it is 

 to be observed, the " biceps-slip" is derived from the coracoid head alone, whereas in the 

 other Procellariidse this slip represents the shorter or humeral head of the normal muscle. 

 Only occasionally have I seen {e.g., in specimens of Procellaria pelagica, Cymochorea 

 leucorrhoa, (Estrelata lessoni, and Prion banksi) a very small tendinous slip derived from 

 this humeral head, which may be either continued downwards with the nerves and vessels 

 to the elbow, where it is apparently lost in the general fascia, or joins the tendon of the 

 true " biceps " (Procellaria, Cymochorea). 



Supposing this latter to represent a more primitive condition, now nearly or epiite lost 

 in most of the species, the biceps muscle must originally have been two-headed, with a 

 patagial slip derived from its humeral head. This slip gradually increased at the expense 

 of the other tendon of the humeral head, till eventually the latter disappeared altogether, 

 the biceps proper (i.e., that flexing the forearm) being then reduced to its coracoidal 

 moiety. 



Expansor secundariorum. — This peculiar muscle 2 is wanting altogether in the Procel- 

 lariidae. It occurs, however, in the Oceanitidse, though in a form different from any 

 previously observed, being attached to (or derived from) thoracically the surface of the 

 pectoralis major muscle (vide PI. III. fig. 3). 



Its small belly is attached to the few last secondary remiges (S.) at the elbow, and the 

 thin tendon (e.s.) runs parallel to, but behind, the humerus, to the axilla, where it is joined 

 by a similar but shorter tendon, which is derived from the most posterior feathers of the 

 humeral tract, the so-called " scapularies" (Sc). The common tendon then runs for- 

 wards, being superficial to the extensor and flexor muscles and the nerves and vessels of 

 the forearm (v.n.), to be attached to the surface of the first pectoral (p. 1) close to its 

 insertion into the humerus. In no other instance, so far as I know, does the expansor 

 secundariorum become thoracically attached to the pectoralis primus, though it may 

 be so to the teres, coraco-brachialis longus, or coraco-brachialis brevis muscles. Nor 

 have I yet met with any other bird in which the tendon of this muscle is connected 

 to the scapularies, which here it serves to expand as well as the secondaries. 



The attachment of this muscle to the pectoralis suggests that the expansor 

 secundariorum may originally have been formed from a cutaneous branch of the former 



1 Cf. Garrod, Coll. Papers, p. 324. 2 Ibid., pp. 323-324. 



