86 FRANK BLAIR HANSON 



being an example of the first, and the pig of the second) ; but none 

 is known in which the relation to both ribs and shoulder-girdle 

 is lacking. Assuming that the sternum must be either a deriv- 

 ative of the shoulder-girdle or of the ribs, it is clearly evident 

 from a phylogenetic viewpoint that since the costae verae ex- 

 tending to the ventral side of the body were not acquired by the 

 vertebrates uutil the rise of the Reptilia, whereas the sternum and 

 shoulder-girdle, in an ever-increasing closeness of relation and 

 association, may be traced back to the very beginning of the 

 Ichthyopsida in the elasmobranchs, that we cannot hope to find 

 in the ribs any clue to sternal origin. If the sternum be homol- 

 ogous throughout, as the conclusions of this present investigation 

 seem to warrant, then its origin may be sought in a structure 

 which is coexistent with it and also in the closest possible rela- 

 tion to it in the lowest forms. In the shoulder-girdle of the Ich- 

 thyopsida we seem to meet with both of these requirements, 

 while in each of them the ribs fail us. 



In Huntington's ('18) paper, which appeared after this work 

 was practically completed, are certain fundamental conceptions 

 of the shoulder-girdle and its phylogenetic relationships which in- 

 directly corroborate my conclusions. In the first place, Hunt- 

 ington recognizes the elasmobranch pectoral girdle as "the 

 primordial fundament upon which all other vertebrate modi- 

 fications are built." By a dual process of segmentation and re- 

 placement by bone, all structures of the complicated girdles of 

 the higher classes of vertebrates are derived from this simple 

 continuous unsegmented bar of cartilage found in the dogfish, 

 sharks or rays. Huntington was not, however, primarily inter- 

 ested in the sternum and did not see in the midventral portion 

 of the elasmobranch girdle the fundament of the presternum. 

 Like other investigators, he finds the "first appearance of the 

 sternal apparatus" in the amphibian girdle, but notes its intimate 

 association with the epicoracoids, which latter cartilages are begin- 

 ning to loosen by sutures on either side of the ventral midline. 

 However, it must be pointed out that these structural relations 

 of the Amphibia (clavicle excepted) are also present in Hexanchus, 

 where a suture on either side of the ventral midline gives the 



