"1. Homologies of parts are best determinable, ceteris paribus, in the most nearly related forms.
"2. Identification should proceed from a central or determinate 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 Ceratodus, 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 comparison in the representatives of the respective classes.
Fig. 71.—Shoulder-girdle of Amia calva (Linnæus).
Fig. 72.—Shoulder-girdle of a Sea Catfish, Selenaspis dowi.
The Girdle in Dipnoans.—"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.