1888 .] 
101 
[Fewkes. 
and homologous with so-called adambulacrals, which are themselves 
simply modified 1 ambulacrals. It may be remembered that adam- 
bulacrals always follow ambulacrals of the same arm joint in their 
time of development. The oral mouth plates are the first formed 
plates of the actinal region of the body and no adambulacrals ante- 
date them in time of formation. As ambulacrals ordinarily pre- 
cede adambulacrals we may regard them as ambulacrals. 
A second fact which looks as if they are ambulacrals is that in 
Asterias they have in the adult a resemblance to ambulacrals and 
have the same relation to the feet. Adambulacrals of the body 
bear, however, both in Asterias and in Asterina, spines which are 
wanting on ambulacrals, but which are present on adambulacrals 
of the arm. This would look as if they might be adambulacrals 
in both cases. 
The time of the relative appearance of plates in Echinoderms is 
not ordinarily regarded as an evidence of homology, and it might 
well be said that the fact that the first orals are the first actinal 
plates to form, while adambulacrals always follow ambulacrals, 
does not prove that the first-formed plates are ambulacrals. 
As far as their resemblance to ambulacrals in Asterias goes, we 
might also say that they also resemble adambulacrals but this ar- 
gument is not conclusive. The objections to both views are impor- 
tant, and while there seems no good reason for calling them one or 
the other it appears that we might regard them the same, and that 
they are calcifications which might well be given a new designation 
for convenience, rather than as an expression of a belief that they 
are morphologically different. I should therefore not accept the 
designation of adambulacrals, given by some naturalists to these 
plates, and I think the mode of their development does not justify 
our accepting the classification of Viguier, who divides starfishes 
into those with an adambulacral and those with a combination of 
ambulacral and adambulacral mouth plates. 
There are other facts to which attention is called in connection 
with those illustrated by fig. 1. The first interbrachial, o (odon- 
tophore), is in the same position in the young Asterias and the 
young Asterina ; the first interbrachial lies between the distal ends 
of two successive ambulacrals, not at the extremity : the first mar- 
ginal bridges the interval between the edges of the terminal and 
the odontophore. These common characters of both genera are 
VThe modification is the result of partial calcification, or rather of calcification 
now in one region, now in another, of a supposed primitive calcareous ring. 
