On the Homologies of the Shoulder-girdle of Fishes. 173 



XX. — On the Homologies of the Shoidder-girdle of the Dipnoans 

 and other Fishes. By THEODORE GiLL, M.I)., Ph.D., &c.* 



Few problems involving the homologies of bones in the ver- 

 tebrate branch have been in so unsatisfactory a condition as 

 that respecting the shoulder-girdle and its constituents in fishes. 

 But the recent observations of Bruhl, Gegenbaur, and Parker 

 have thrown a flood of light upon the subject. Some minor 

 questions, however, appear still to be unsettled ; the writer, at 

 least, has not been able to convince himself of the correctness 

 of all the identifications, and of the names conferred as ex- 

 pressions thereof. Recent study has increased such doubts 

 respecting the applicability of former nomenclatures, and has 

 led to conclusions different from those announced by previous 

 investigators. 



The following are assumed as premises that will be granted 

 by all zootomists : — . 



1. Homologies of parts are best determinable, cceteris pari- 

 Z>Mst> in the most nearly related forms. 



2. Identifications should proceed from a central or determi- 

 nate point outwards. 



The applications of these principles are embodied in the 

 following conclusions : — 



1. The forms that are best comparable and that are most 

 nearly related to each other are the Dipnoi, an order of fishes 

 at present represented by Lepidosiren^ Protopterus^ and Cera- 

 todus, and the Batrachians as represented by the Ganocephala, 

 Salamanders, and Salamander-like animals. 



2. The articulation of the anterior member with the shoulder- 

 girdle forms the most obvious and determinable point for com- 

 parison in the representatives of the respective classes. 



The Girdle in Dipnoans. 



I. The proximal element of the anterior limb in the Dipnoi 

 has, almost by common consent, been regarded as homologous 

 with the humerus of the higher vertebrates. 



II. The humerus in the Urodele Batrachians, as well as the 

 extinct Ganocephala and Labyrinthodontia, is articulated 

 chiefly with the coracoid. 



Therefore the element of the shoulder-girdle with which 

 the humerus of the Dipnoi is articulated must also be regarded 



* Abstract, communicated by the Author, from a forthcoming work 

 (' Arrangement of the Families of Fishes ') now being printed for the 

 Smithsonian Institution. 



t Parts affected by teleological modifications may be excepted. 



