:Fewkes.] 
100 
[Dec. 5, 
tions. We have in addition, a terminal plate, £, and a single 
marginal, m, on each side of the arm. A single interbrachial, o, 
(odontophore) is also represented. This stage in the growth of the 
Asterina arm differs considerably from the oldest figure by Lndwig. 
Several of these differences are worthy of special mention. 
Fig. 1 is believed to be the first correct representation of the 
relation of the actinal plates in an Asterina of this age. Some of 
the differences between it and a similar stage of Asterias are as 
•follows : 
The oral adambulacrals of Asterina. — The so-called oral adam- 
bulacrals, ado , extend from the interradii to the medial actinal line 
of the arm. 
These extensions are regarded as the adambulacrals and not sep- 
arate calcifications, but are thought to be the same as similar cal- 
cifications in Asterias, where they are, as elsewhere 1 shown, simply 
the ends of the ambulacrals. 
They differ here, as in Asterias, from other ambulacrals, and seem 
to form like the spoon-shaped plates of Amphiura. These ambu- 
lacrals are believed to be homologous with the first formed oral 
ambulacral plates of Asterias, although in this genus they take on 
the form of ambulacral rafters, which they never assume in Aste- 
rina. These oral plates of both genera are thought to be the same 
plates differently modified in form, but it is not necessary to sup- 
pose that any new plates are thus far introduced in the formation 
of the calcifications of the mouth. In other words it is believed 
that the mouth plates of two representative starfishes are built es- 
sentially on the same plan, and that a division of starfishes into 
those with adambulacral and those with ambulacral mouths is not 
found in nature. A second character of starfishes with an ambu- 
lacral mouth is that the feet have a biserial arrangement. This is 
an adult character of Asterina, although a larval feature of Aste- 
rias as several naturalists have already shown. 2 
It can then be shown that the plates of the mouth of Asterias 
and Asterina are essentially similar in mode of formation but it is 
more difficult to say whether these oral plates are homologous 
with the other ambulacrals or adambulacrals of the arm. I believe 
that they are homologous in both cases with ambulacrals, and that 
they may be regarded as the distal extremities of the ambulacrals 
1 In my paper on the calcareous plates of Asterias, op. cit. 
2 Bull. Mus. Comp. Zool., Vol. XVII, No. 1. 
