400 The American Naturalist. - [May, 
the “intercentrum ” as the enlarged base of the lower arches, 
instead of a complete vertebral body. 
From the consideration of the vertebrae of Amia from this 
point of view Schmidt turns to the discussion of its fossil rela- 
tives; and here he gives a different solution of the problem, 
the one that Goette endeavors to have us accept as the only one 
proposed by his pupil. The “intercentrum” is regarded as 
increasing at the expense of the “ centrum,” until in Megalurus 
and Amia the latter element is reduced to a mere vestige, the 
“rudimentary upper arch.” The first method of union is that 
of two nearly equal elements, the latter the union of the lion 
and the lamb—with the lamb inside of the lion. 
So far from being open to the charge of opposing the view 
that the dorsal vertebre are constituted of elements homolog- 
ous with those found in the somites of the tail, with reduction 
of some of them and expansion of others, that is the very pith 
of the embryological portion of my former paper. 
I may remark here that in Amia the pleurocentral element 
can hardly be regarded as vestigial, since it appears to furnish 
the foundation for the upper half of the vertebral centrum. 
Not merely the small portion of the cartilage which is seen in 
front of the upper arch, but the whole of the cartilage in the 
upper half of the dorsal centrum, belongs to the pleurocentrum. 
The figures of Callopterus reproduced by Goette are very inter- 
esting, inasmuch as they show to what extent in that form the 
above element had become reduced. When the reduction be- 
comes complete, the vertebral centrum becomes a hypocentrum, 
a condition affirmed by Cope to be that of the higher fishes in 
general. 
Aside from any misunderstandings, there exist between 
Goette and myself certain differences which are fundamental. 
He holds that there is to be found in most vertebrates a spe- 
cialized sheath which, composed of cells, surrounds the noto- 
chord and becomes segmented to form the “primary vertebral 
centra.” This sheath, called in other publications “äussere 
oder zellige Chordascheide,” is now named the “ perichordal 
sheath.” In some of Goette’s writings it appears to be certain 
-that he has in mind the elastica externa of other writers; in 
