No. 2.J AM/A AND TELEOSTS. 



91 



But even if this difference in the morphological relations of the 

 nerves to the vertebrae were evident in the skull alone, Scomber 

 would still present a marked exception to Fiirbringer's general 

 formula ; for, if the most anterior spino-occipital nerve of this 

 fish is considered as nerve b of his nomenclature, the most 

 posterior one would necessarily be nerve d, and not nerve 4 ; 

 and such a nerve is not given, or its existence intimated, in 

 any of the teleosts considered by him. If, on the contrary, 

 the most posterior nerve is to be considered as nerve c, the 

 most anterior one would be nerve a ; a nerve said by him to be 

 absolutely wanting in all teleosts. 



The successive incorporation of vertebrae in the occipital 

 part of the skull is attributed by Fiirbringer, primarily (unmit- 

 telbar), to the reduction and disappearance of the myomeres 

 that give to the vertebrae in question their movements relative 

 to each other and to the skull (No. 2, p. 548). This same 

 reduction and subsequent disappearance of the anterior muscle- 

 segments is also said to precede and be the primary cause of 

 the reduction and disappearance of the nerves related to them 

 (No. 2, p. 543). 



Why, then, is there, in the adult of both Scomber and Amia, 

 an anterior muscle-segment, relatively well developed, without 

 any indication whatever of a separate spinal-like nerve related 

 to it } And why is it that in Amia the last so-called occipital 

 vertebra is incorporated in the skull after the fish has passed 

 the age represented by a 50 mm. specimen, and yet, between 

 the age represented by a 12 mm. larva and the adult fish, there 

 is no related reduction in the number of myotomes } As there 

 are, both in the adult and in larva, four muscle-segments ante- 

 rior to the one that, in the adult, lies between the last assimi- 

 lated vertebra and the next anterior one, some reduction in this 

 number might have naturally been expected. In Acipenser 

 ruthenus, according to Sewertzoff (No. 7, p. 232), there are 

 always, in the adult, two or three spino-occipital nerves anterior 

 to the one that innervates the most anterior myotome. 



The temporal extensions of the trunk muscles certainly rep- 

 resent to some extent, in Scomber and in Amia, independent 

 invasions of the cranial region, for in Amia these muscles lie 



