JTewkes.] 
98 
[Dec. 5, 
fishes and sea urchins will at once be recognized when I mention 
the fact that many morphologists consider the ambulacral plates 
in Asterias, which are above or internal to the water-vascular sys- 
tem, as homologous to plates of the sea-urchin which are outside 
the same. An homology of plates in such morphologically different 
positions, one set external, the other internal, presents many diffi- 
culties, and it is thought that the morphology suggested in the 
following pages offers a better solution of the whole question of 
the relationship of these calcifications than that commonly given. 
A key to this solution is found in a study of the plates of the 
mouth. 
In the oldest form of the Asterina young which Ludwig has fig- 
ured (Pl. viii, fig. 106) we find that there are represented on the 
abactinal side of this starfish the following plates : 
1. Dorsocentral. 
2. Five terminals. 
3. Five genitals, or basals. 
4. Five first dorsals. 
5. Ten marginals. 
In a more or less schematic figure of a similar stage (p. 72), giv- 
en in his text we have in addition : 
6. Five unpaired ‘‘Intermediate ” (odontophores). 
7. Thirty ambulacral rafters. 
8. Thirty adambulacrals (interambulacrals). 
In the latter figure, which is diagrammatic, the oral plates are 
represented together with the aboral and are affixed to the ends of 
the rays. 
It will be seen from the list of plates mentioned that Ludwig has 
traced the origin of the majority of the primary plates of the body. 
There are, however, several remaining plates of the origin of which 
nothing is yet published. Let us take up the subject of the growth 
of these plates where he has left it, and trace the origin of certain 
other late formed calcifications which play an important role in the 
form of the adult of the Asterina. Our account especially concerns 
the adambulacrals, the dorsals of the arm, the sub-basals, the mar- 
ginals and some others. 
Two divisions of the Asteroids have been made, based upon other 
characters than the morphology of the oral plates, yet although 
other features, as the number of rows of legs, and the character of 
the pedicellarise are also used in combination with the character of 
