on the Genus Solaster. 439 
we have subjected the parts of the skeleton to a very careful 
examination, and we shall now state what we have found with 
regard to the marginal plates. 
In Solaster endeca there are two rows of marginal plates, 
one belonging to the dorsal, the other to the ventral surface. 
They are concealed by the thick skin, and are observed only 
when this, together with the paxille, is removed. 
The dorsal marginal plates are flattened, and have a trian- 
gular form, with the narrower part turned outward; on the 
lower surface of this narrower part there is a small button-like 
elevation, which forms the articular surface for the paxilla. 
The broader part is directed inwards, and rests with its slightly 
concave superior surface upon the corresponding ventral mar- 
ginal plate, while the slightly convex inferior surface contri- 
butes, in conjunction with the ventral plate, to form a common 
point of support for the broad transverse row of paxillee which 
is nearest the papille of the groove. 
The ventral marginal plates are likewise flattened ; they are 
somewhat broader than the dorsal, but, like these, have a 
triangular form, of which the broader part is turned outwards, 
and forms with its lower surface a point of support for the 
dorsal marginal plate. The smaller part of the ventral plate 
is turned inward; its upper surface is concave, and rests for 
the most part against the lower rounded margin of the adam- 
bulacral plate (in the interbrachial space it is in contact with 
one of the ventral plates) ; its lower surface is slightly convex, 
and, in conjunction with the broader part of the dorsal margi- 
nal plate, forms a sort of articular surface for the before-men- 
tioned transverse row of paxille. 
In the broader parts of the arms and in the angles of the 
arms both the dorsal and ventral marginal plates stand a little 
apart from each other; but they become closer and closer to- 
gether as they approach the ends of the arms. 
Solaster papposus has also two rows of marginal plates, 
which are likewise well covered, but differ essentially from 
those of iS. endeca in bearing only one row of paxille, while 
in S. endeca each bears its own, and there are therefore two. 
The dorsal marginal plates in Soluster papposus are rather 
small, oval in the upper part of the arm, while in the middle 
and in the narrow part they are nearly cubical. When they 
bear a paxilla, two stand together and form with their outer 
ends a concave articular surface, which receives the large 
paxilla forming the boundary between the dorsal and ventral 
surfaces; very often, however, the inner end of a calcareous 
trabecula is contiguous to the above-mentioned articular sur- 
face, as if to complete it for the large articular head of the 
