394 ON THE PETRELS COLLECTED DURING 



that it arises independently 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 Procellariidae this slip represents the shorter or humeral head of 

 the normal muscle. 



Only occasionally have I seen (e. g. in specimens of Procdlaria pelagica, 

 Cymochorea leucorrTioa, (Estrelata lessoni, and Prion banksi) a very small 

 tendinous slip derived from this humeral head, which may be either con- 

 tinued 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 quite lost in most of the species, the biceps muscle must origi- 

 nally 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 * is wanting altogether 

 in the Procellariidse. It occurs, however, in the Oceanitidae, though in a 

 form different from any previously observed, being attached to (or de- 

 rived from) thoracically the surface of the pectoralis major muscle (vide 

 PI. XVII. fig. 2). 



Its small belly is attached to the few last secondary remiges ($.) 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 " ($c.). The common tendon then runs 

 forwards, 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 origin ally have been formed from a cutaneous 



Zool. Chall. branch of the former similar to others of the same function derived from 



Exp. *<*** it, which are still broad thin expansions of muscular fibres. In other 



* 7Z.irf.pp. 323, 324. 



