The simplest form of a diverging appendage is seen in the thorax of birds, from whose pleurapophyses, or ribs, there extend backwards short flat plates of bone, which overlap the external surface of the pleurapophysis, or rib, behind. The anterior and the posterior extremities are the highly-developed pleurapophysial appendages of the occipital and of the first sacral vertebras ; the former retaining its typical connection in fish, but displaced in most Vertebrata, with its pleurapophysis and hsemapophysis, to the upper dorsal region. In the flat diverging pectoral fin of the cod or the dolphin, we see a repetition of the diverging costal appendages of birds ; similar to them in shape and direction, but composed of the same primitive pieces, with the occasional exception of the humerus, as are noticed in the upper extremity of man.
The flat short bones of the upper extremity, in fish, become elongated in air-breathing Vertebrata in relation to the more varied and extended movements which they have to perform. The humerus, or the first bone of the appendage, is articulated to the pleurapophysis; and the succeeding bones, increasing in number, acquire one with another a degree of mobility, less in extent, but more definite and certain in character. It is erroneous to connect with the word humerus the idea of a long straight cylindrical bone: in the turtle the humerus is S-shaped, and surmounted by its two tuberosities; in birds it is a long cylindrical bone, as in man ; in the mole it is expanded into a broad flat plate, twisted upon itself, and notched above and below for the transmission of arteries. In the same animal, the scapula, which we are in the habit of regarding as a flat bone, is elongated, and comes more under the head of a cylindrical bone than do any of the bones of the anterior extremity.