230 E. Billings on the structure of Crinoidea, Cystidea, ete. 
In the next series there are five plates which are mponieey 
the homologues of the five forked plates of Pentremites. 
are very short and confined to the base of the Tals. They 
form a shallow basin with ten re- entering angles in its margin. 
6, 
ig. 
‘Alternating above the forked af pate are five pieces corres- 
ponding to the deltoid or interradi of Pentremites. Some 
of these are lanceolate in form (fig. 6, d), their broader extremi- 
ties fitting into the angles betraar the forked plates. They 
taper to a point upward, and their sides are bevelled so as to 
pass under the am alacral age to which they are, in general, 
so closely united, that the line of junction is indicated paly by 
the difference in the markings of the surface. Owing to this 
structure, these plates have not always been recognized a the 
authors who have described this g enus. They were first point- 
eltoid or interradial plate is 
(mv, figs. 4, 6). The sutures on each side of this plate are gen- 
“aid “distinctly es especially in the upper part of the 
kav = two rows of pores, those on one side of the groove 
altern rie a in position with those on the other side. These 
into the hydrospires. There appear to be only two 
rows of ambulacral ossicles. The pores are situated in the 
sutures between them. On each side of the ambulacrum there 
is a broad erie k grooved marginal plate. From each pore 
a small runs across this plate. e grooves 
between the ridges originate at the outer extremities of the 
ambulacral ossicles. In well-preserved specimens the surface 
of these marginal plates exhibits no other structure than the 
verse grooves and ridges; but in one weathered specimen 
that I have examined, they seem to be composed of a number 
of narrow elongated pieces, arranged transversely, in such a 
manner that two of them abut pk. the outer extremity of 
each of the ambulacral ossicles, and extend outward toward 
the interradials. This seems to prove that the shape» lates 
I one mae a large ae of specimens, none of be 
Pans: seen enough to convince me that the ambulacra are 
more com Re is usually supposed. The lancet plate, if it 
occur at all in this genus, must be very narrow. The am 
eral groove, as in Pentremites, sends off branches, right and left 
