TO BIRD-ANATOMY AND CLASSIFICATION. 201 



extremity of the furcula, whilst their tendons run between the two 

 layers of membrane of which the patagium is composed. These muscles 

 are the tensor patagii longus and the tensor patagii brevis. Arising in 

 common, or in close proximity to each other, the tendon of the tensor 

 patagii longus forms the more or less stiffened, though flexible, anterior 

 border of the patagium ; the tendon of the shorter of the two muscles 

 runs more or less parallel with the humerus, ending near the elbow-joint. 

 In different birds its insertion takes place in different ways, the tendon 

 in some cases simply running straight on to the ulnar side, and there 

 becoming blended with the general fascia of the part, whilst in others 

 it becomes united, more or less intimately, with the tendinous origin, 

 springing from a tubercle on the humerus, of a muscle lying on the 

 radial side, the extensor metacarpi radialis longior. In some cases 

 additional slips from the main tendon are given off, the arrangement 

 sometimes thus getting very complicated (e. g., in the Trogonidae). As a 

 rule, every natural family of birds has a characteristic arrangement of 

 these tendons ; so that their disposition often affords great help in classi- 

 fication. This is the case, for example, in the Passeres ; throughout the 

 whole of that immense group one arrangement, only slightly masked in 

 one or two aberrant forms, which can be easily recognized and is most 

 characteristic, obtains. The same is the case with the very closely allied 

 groups of Picidae, Eamphastidae, Capitonidae, and Indicatoridae. These 

 points were first worked out by Prof. Garrod in the first of his papers 

 on Passerine birds * ; and the arrangements in nearly all the families 

 of his " Anomalogonatous birds " are there described and in several cases 

 figured. In the Homalogonatse, too, they offer nearly as well-marked 

 peculiarities, though, unfortunately, his purpose of describing and figuring Ibis, 1881, 

 their arrangement in these birds was never carried out. In his MSS. and p ' 17t 

 drawings, however, he has recorded their condition in very many groups. 



VII. Certain other Muscles. 



In his paper on Chauna (supra, p. 197), Prof. Garrod, for the first 

 time, called attention to the value in classification of certain other 

 myological facts. These are : 



(1) The presence or absence of the '* cxpansor secundariorum " muscle. 



This is a slender muscle which arises from the last few (generally two 

 or three) secondary quills and has a peculiarly long and slender tendon, 

 which, running superficially posterior to the humerus, together with the 

 axillary vessels and nerves, is inserted into the thorax in different ways 

 in different birds. One common arrangement is that found well deve- 

 loped in the Storks, and hence called " ciconiiform." Here the proximal 

 part of the tendon is T-shaped, it splitting into two parts, one being 



* P. Z. S. 1876, pp. 506-512, pis. xlviii.-li. 



