No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 719 
the study of a 57 mm. Amia, I can form no opinion as yet. 
It is to be noted, however, that they consider the precentrum, 
or arch-bearing element of a complete vertebra, the intercen- 
trum, therefore, of Schmidt, as the anterior of the two elements 
of a complete vertebra, instead of the posterior. This anterior 
element of the complete tail vertebra is described by them 
as bi-protovertebral or bi-myomeric in origin, being formed by 
the fusion of “the basidorsal of one sklerotome and the basi- 
ventral of the next previous sklerotome.’”’ The posterior, arch- 
less disk of a complete tail vertebra, the postcentrum, is formed 
by the interdorsalia and interventralia of the same sklerotome. 
“The intermuscular septum runs obliquely across the precent- 
rum.” The complete trunk vertebrae are thus, according to 
them, partly bi-myomeric, instead of being myomeric, as 
Schmidt finds them ; and the dorsal arch that belongs to any 
particular vertebra must be, in 57 mm. specimens, and hence 
also in the adult, the one that lies in front of that vertebra 
instead of the one that lies behind it. Gadow and Abbott, 
therefore, assign the arches to the vertebrae next posterior to 
the ones to which Schmidt assigns them. Schmidt’s con- 
clusions in this respect agree with the earlier statements of 
Sagemehl (No. 104) and Ihering (No. 61). 
Although my own work as yet warrants no opinion on this 
subject, I cannot avoid the impression that each “ Anlage”’ of 
a dorsal arch, arising, as it is said to, from parts of two adjoin- 
ing somites, must, because of this double origin, have inherently 
in itself the possibility of forming two separate pieces ; and 
that these two pieces may remain separate, giving origin to 
two arches, or that they may fuse, or that one of them may 
disappear, thus giving origin in either case to a single arch. 
The single arches so formed would doubtless always remain 
septal at their bases ; they would, however, tend, because of 
their different origin in different species, to occupy different 
positions on, or relative to, their vertebrae. Those vertebrae, 
also, even in nearly related forms, may not be homologous 
structures, for Gadow and Abbott (No. 39, p. 298) state that 
the vertebrae of Lepidosteus are truly bi-myomeric, while those 
of Amia are not so. 
