ONTOGENY AND PHYLOGENY OF THE STERNUM 45 



3. Work of Parker and Howes 



Parker ('91) claims to have found a sternum in the shark 

 Notidanus indicus. A small blunt process is set in between the 

 two cartilages which unite later to form the girdle. This struc- 

 ture was earlier described in the same shark by Haswell ( '84) who 

 says "the intercepted cartilage is temptingly like a presternal, 

 but the absence of such an element in the skeleton of any group 

 nearer than the Amphibia seems to preclude this explanation." 

 Parker's ('91) figures 1 and 2 would indicate that this was a 

 presternum, and that Haswell was more nearly correct in his 

 observation than in his deduction therefrom. Had Paterson used 

 Hexanchus rather than Acanthias, he might have found an even 

 more striking resemblance of stages between the rat and shark 

 than he did. 



By the courtesy of the officials of the U. S. National Museum, 

 I was permitted to examine their type specimen of Hexanchus. 

 The body wall had been laid open along the ventral side to allow 

 the preserving fluid to bathe the viscera. By means of a short 

 anterior and two lateral incisions I was enabled to lay bare the 

 median ventral portion of the pectoral girdle without otherwise 

 disturbing the value of the specimen as a type. The girdle 

 (fig. 1) was exactly as described by Haswell ('84) and Parker 

 ('91). The median cartilage in a young specimen was dis- 

 tinctly marked off from the coracoids and in general appearance 

 was not unlike the fetal girdle of the marsupial (fig. 35). Later 

 I dissected two specimens of Acanthias that measured 4 inches 

 and 7 inches, respectively, in order to confirm Paterson's 

 statement of its likeness to the early embryonic girdle of the rat. 

 In both these specimens the girdle was approximately the same 

 as in the adult. Figure 2 shows the girdle of the 7-inch speci- 

 men, and if compared to the marsupial girdle (fig. 35) and the 

 mouse girdle (fig. 5), the morphological relations are apparent. 

 In these early stages of the marsupial and mouse no suture has 

 as yet appeared between coracoids and presternum, giving the 

 resulting shark-like girdle, complete across the midventral line. 



