160 



STRUCTURE OF THE VERTEBRATES 



to the girdle of this animal, the clavicle, parallelling the cora- 

 coid on its anterior face, and also meeting its mate at the mid- 

 ventral line. It is unknown if this bone is of cartilage or dermal 

 origin in the fish. In the frog it is of mixed origin, and in the 

 higher forms the dermal part is apparently lost. 



Ulna 





D 



Fig. 96. (A) Shoulder Girdle and Appendage of Eryops (Order Stegoce- 

 phalia), an extinct amphibian with a primitive girdle and a heavy, short 

 humerus. (B) The foot-print of Thinopus, the earhest record of land 



animals. 



The crossopterygian girdle was carried over to the earliest 

 amphibia almost in its entirety. The ventral clavicles and cora- 

 coids met in the midline; and the scapulae were connected with 

 the head by small dermal bones. The latter were soon lost, for 

 the connection was a liability under conditions where flexibility 

 between head and shoulder were required. 



