CAtTDAti DIPLOSJONBTLt OF SHARKS. 



m 



inexplicable without haviug recourse to the former is a sign of 

 weakness in the argument. And besides, Dr. Gradow is here 

 disregarding his own word of caution expressed on page 193 of 

 his treatise, "Indeed, mischief enough has been done by the 

 selection of the Eajidas for the elucidation of fundamental 

 morphological questions." 



Much may be said in favour of the contention of Mayer and 

 G-adow (13. p. 266, and 4. p. 195) to restrict the word " vertebra " 

 or " spondylus " to a complete scleromere, equal in value and 

 antero-posterior extent to a neuromere and myomere ; although 

 to agree in this is not necessarily to accept the conclusion of the 

 latter writer that " diplospondylous " is a " term without any 

 reasonable meaning." For, after all, these " vertebrae " of the tail 

 of Sharks are so regular and complete that we can scarcely deny 

 them the title. Each consists of a centrum of cartilage, partially 

 calcified, with a conical depression in front and another behind, 

 occupied by persistent notochord. Above each centrum, and united 

 with it, is a pair of cartilaginous plates, and between every two 

 consecutive "vertebrae" a pair of intercalary plates, while located 

 over the intervals between these alternating plates are median 

 dorsal cartilages, twice as numerous as the centra. That is to 

 say, the structure of each " vertebra " of the tail is exactly the 

 same (neglecting, of course, the distinctive features such as 

 haemal arches and absence of ribs) as that of a trunk vertebra, 

 except that every alternate one has no nerve foramina, wdiile all 

 the trunk vertebrae are provided with them. 



If, therefore, we deny the caudal " vertebrae " the right to 

 rank as equal to those of the trunk region because of their failure 

 to fall in with the metamerism of the muscular and nervous 

 systems, we must yet admit for them a metamerism of their own, 

 which is almost as perfect as that of the trunk vertebrae. It is 

 certain that there is no such " discrepancy between chorda centra 

 and arches," or " difference between the metamerism of the 

 centra and that of the arches," as Dr. Gradow would have. The 

 only discrepancy occurs in the transition region, where it could 

 hardly be avoided ; and even there it only affects four or five 

 segments of the body. 



Von Iheriug conceived the idea (9. p. 235) that in the primi- 

 tive Selachians the whole vertebral column was diplospondylous, 

 and that the monospondylous condition is secondary, and has 

 been introduced by a fusion of parts proceeding regularly from 



