THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 79 



fin. These fin folds were supported by cartilaginous fin rays. The paired fins of present 

 fishes are supposed to have arisen through the persistence of certain regions of the paired fin 

 folds, and the median unpaired fins through the persistence of particular regions of the median 

 fin fold. The remaining portions of the fin folds have vanished (see Fig. 23$). This theory 

 is supported by the facts that the unpaired and paired fins of fishes are identical in structure, that 

 in the young stages of some forms continuous paired folds are present from which the paired fins 

 arise, and that in the extinct shark Cladoselache (see K, Fig. 121, p. 115) the arrangement of 

 the fin rays in the paired fins is such as to suggest strongly the origin of the paired fins from 

 continuous folds. 



The girdles arose later than the paired fins, since extinct forms possess the latter without 

 the former. The fins as already stated are supported by cartilaginous fin rays, which may 

 occur in several rows. It is suggested that the most medial and anterior of these fin rays 

 of the fins fused across the midventral line to form median plates or bars of cartilage. Such 

 plates or bars represent primitive girdles. Later ossification occurs in the cartilage, result- 

 ing in the bony girdles composed of several bones, as found in the higher vertebrates. 

 Read W. 



In the evolution of fish into Amphibia the paired fins transformed into limbs. Stages 

 in this transformation are very imperfectly known and difficult to conceive. It is supposed 

 that the fin rays of the paired fins were gradually reduced in number either by loss or fusion. 

 A few enlarged fin rays arranged similarly to the bones of the limbs persisted. Such reduc- 

 tion and enlargement of fin rays really occurs in the crossopterygian ganoids living and 

 extinct, and the arrangement of the fin rays in these forms is strongly suggestive of the 

 arrangement of the bones in the vertebrate limb. (See further, N, p. 176, Fig. 102, and 

 p. 178, Fig. 103.) 



Read further on the comparative anatomy of the sternum, girdles, and paired append- 

 ages in K, W, and Wd. 



B. THE PELVIC GIRDLE AND THE POSTERIOR PAIRED APPENDAGES 



1. The parts of a typical girdle and hind limb. Before beginning the study of the 

 comparative anatomy of these parts it may be well to describe the generalized girdle and 

 limb. The bony pelvic girdle arises from the ossification of a cartilaginous arch or plate. 

 Three centers of ossification appear in each half, resulting in the production of three pairs of 

 cartilage bones. Of these there are on each side a dorsal bone the ilium -an anterior 

 ventral bone the pubis and a posterior ventral bone the ischium (Fig. 24). The two 

 pubes and ischia commonly meet in the median ventral line. The ilia are articulated to the 

 sacral ribs. 



The hind limb typically consists of three regions: a proximal segment, the thigh, containing 

 a single long bone, the femur; a middle segment, the shank, containing two long bones: a pre- 

 axial tibia and a posta.xia.1 fibula; and a distal segment, the foot or pes, including the ankle, sole, 

 and toes, containing a number of small bones. The generalized ankle is composed of 

 nine or ten bones in three rows: a proximal row of three bones a preaxial tibiale, a medial 

 intermedium, and a postaxi&lfibulare; a middle row of one or two centrales; and a distal row of 

 five tar sales. The sole of the foot is composed of five bones, the metatarsals, arranged in a trans- 

 verse row. The toes or digits contain a series of small bones, the phalanges, of which in 

 primitive feet there are two to the first digit and three to all of the others. Figure 24 illus- 

 trates the bones of the typical pelvic girdle and hind limb. 



2. The pelvic girdle and pelvic fin of elasmobranchs. For this purpose 

 study preserved skeletons of the dogfish or other elasmobranch fish. The pelvic 



