1888 .] 
103 
[Fewkes. 
in the other of ambulacrals and adambulacrals, but that they are 
really in both cases homologous. 
The marginals of the pentagonal starfishes, as we might expect 
from their relation to sea-urchins, reach a much greater develop- 
ment than the same plates in stellate genera. This is what we 
should find if these are nearer the Echinoids since I look to the 
marginals for the homologues of plates in the urchins formerly re- 
garded as adambulacrals. In the starfishes these plates form along 
the sides of the arms although, as in Asterina, and other pen- 
tagonal starfishes, the interval between them is often filled up 
by a number of interbrachial plates. In sea-urchins, however, the 
interbrachial plates fail to form between marginals, and the margi- 
nals of adjacent rays are not separated but always remain in con- 
tact. The formation of interbrachials in starfishes of a pentagonal 
form is manifestly necessary on account of the rapid growth of the 
arm in stellate directions. In Palmipes, the interbrachials are 
enormously developed while in Asterias they are small in number 
and rudimentary. In Echinoids, as a general thing, they are want- 
ing. 
The later stages of formation of the pentagonal form of Aster- 
ina result frequently from the interposition of new marginals placed 
outside the marginals already formed. These originate between 
those already formed and the interambul acral, and the first pair 
arise near the interbrachial ends as represented. The second in- 
terbrachial originates as in Asterias. The marginals are centripetal 
i. e., the younger are nearer the axis than the older, not as is the 
case in the ambulacrals and dorsals, centripetal, i. e., those nearest 
the terminal being the latest found. 
In Asterias I have shown that the body plates between the ter- 
minals and dorsocentral form before those which lie between the 
genitals and dorsocentral. In Asterina the plates between the 
genitals, 6, and dorsocentral, form before those on the radial lines. 
I expressed a doubt in my paper on Asterias whether these plates 
ought to be called underbasals. In Asterina we might call them 
underbasals and suppose that they are belated in Asterias. We 
may say, considering the fact that the pentagonal form is that to 
which the development of the young Asterina is tending by virtue 
of the growth of the plate of the actinal region, that we may thus 
explain the early appearance of these plates. 
From what has been given it follows that the so-called inter- 
