78 STRUCTURE AND CLASSIFICATION OF BIRDS 



of the total series of muscles in the body that can be trusted much 

 as evidence of affinity. The ambiens is unquestionably of value as 

 it is found or not found, as the case may be, through whole groups 

 whose mode of progression when walking or climbing is as different 

 as can be. Its total absence from all picarian and passerine birds 

 is a fact upon which I comment elsewhere. There are very few 

 groups in which the ambiens may be present or absent, and in 

 those cases it is often reasonable to separate as distinct families 

 the genera which have it from those which have it not. This 

 cannot, perhaps, be done in every case. Some storks, such as 

 Abdimia, have no ambiens, while the majority have it. There are 

 auks with and auks without this muscle. The same may be said 

 of petrels, parrots, and pigeons. Rlu/nclwps, the only larine bird 

 without an ambiens, may be, perhaps, rightly elevated to the dis- 

 tinction of a separate family. These examples, however, are so 

 few that they may be compared to such singular exceptions as the 

 absence of the odontophore in the nudibranch Doriopsis, which 

 does not in the opinion of any one invalidate the great importance 

 of that structure in arranging the mollusca. In estimating the 

 value of the ambiens the facts of its total or apparently partial 

 suppression, referred to below, must be borne in mind. The 

 entire absence of all trace of the muscle in the owls shows that 

 they are not necessarily to be placed in the neighbourhood of the 

 parrots, in which the muscle, when absent, has left traces behind. 



Muscles of the Fore Limb 



Pectoralis Primus. This muscle consists of two parts, 

 the thoracic part, arising from the sternum, and an abdominal 

 portion, arising from the pelvis. The latter portion, well 

 developed in lower vertebrates, is slight in birds, and is often 

 completely absent. The pectoralis thoracicus arises from 

 the sternum, the clavicles, and intermediate membranes ; it is 

 inserted on to the humerus. In ratite birds there is no origin 

 from clavicles, but, on the other hand, an origin from cora- 

 coids not present in carinates. There is frequently an 

 intimate connection between the pectoral near its insertion 

 and the tendon of origin of biceps. The pectoralis is frequently 

 divided into two portions, the mode of division being twofold. 

 In Apteryx the coracoidal portion is separate from the sternal, 

 a state of affairs which recalls some of the lower vertebrates ; 



