REPORT ON THE ANATOMY OF THE PETRELS. 29 
In nearly all the other Procellariide, 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 patagi longus tendon, which it joins as a short, cylindrical tendon close to 
the shoulder (Pl. III. fig. 4, 2.). It is, therefore, functionally a “ biceps-slip,” though it 
differs from the ordinary “biceps-slip” found in so many birds," 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 Procellariide this slip represents the shorter or humeral head of the normal muscle. 
Only occasionally have I seen (e.g., in specimens of Procellavia pelagica, Cymochorea 
leucorrhoa, Gistrelata 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 quite 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 (7.e., that flexing the forearm) being then reduced to its coracoidal 
moiety. 
Expansor secundariorum,—This peculiar muscle? is wanting altogether in the Procel- 
lariide. It occurs, however, in the Oceanitide, though in a form different from any 
previously observed, being attached to (or derived from) thoracically the surface of the 
pectoralis major muscle (vide Pl. 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.7,), 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 Thid., pp. 823-324. 
