CALYPTOCRINID &. 333 
(Sr 
than the second. The first palmars support the arms; they are short, trun- 
cated at the upper face, and are provided with small muscle plates and 
fossee. First interbrachials generally the largest plates of the dorsal cup, 
more than twice as large as the two above. The two latter are of about the 
same form and size, and are in contact laterally to their full length. They 
rise some distance beyond the top of the palmars in square or tapering trun- 
cate projections, and jointly form a support for the interradial partitions. 
The interdistichals rest either within notches formed by the distichals, or 
upon the truncate upper face of the second costals. They resemble in form 
the combined upper interbrachials, rise like these to the height of the third 
arm plates, and support in a similar manner to the interbrachials a partition 
wall upon their truncate upper faces. The projections between the arm 
sockets give to the specimens a very marked aspect, and serve as a reliable 
guide for generic identification in case the ventral structures of the calyx 
are not preserved ; in that condition they look like cogs of a wheel. 
The tegmen, or part above the dorsal cup, consists of four rings of plates 
of peculiar structure. The lower ring is composed of five elongate inter- 
radial pieces which rest upon the projecting upper faces of the interbrachials, 
and five plates of similar form and size supported by the interdistichals. 
The middle portions of the ten plates extend outward and upward in form of 
knife-like winged processes, of which the upper end rises to near the fourth 
row of plates, overlapping the plates of the second and third rows, to the 
outer faces of which they are attached by suture. Interposed between these 
plates are ten small trigonal pieces, arranged in such a manner that a plate 
supported by an interbrachial, and one supported by an interdistichal, always 
meet laterally over the top of one of the triangular pieces. The latter plates, 
which are somewhat thickened at their median lines, form a sort of subpar- 
titions between the arms of the various pairs, without being visible when the 
arms are intact. Their edges at both sides, and also those of the larger 
plates, are pierced by the ambulacral or arm openings, which are well 
defined in this genus, and enter the calyx between the two plates. The 
second and third rings, which form the tubular prolongation of the calyx, 
are composed of four plates each; the fourth ring consists of ten pieces, 
which meet around the summit. The plates of the second ring slope 
upwards; two of them are wider, and alternate with the other two; the 
two narrower plates are longer, and angular at the top, the others truncate. 
United they form an inverted funnel, the margin surrounded by ten protu- 
