PLATYCRINIDZA. 665 
trigonal, the outer end of the first distichal barely touches the radials. Dis- 
tichals placed obliquely, about as long as half their width, the axillary 
obtusely angular above. ‘There are from six to nine higher orders of 
brachials above the costals, of which the proximal ones consist of two 
pieces each, the succeeding ones of three each, all giving off from their 
axillaries at one side an arm, and from the opposite side the next order 
of brachials. The plates of the different orders decrease in width upward, 
but not in length, those of the higher orders being almost as long as wide, 
while those of the lower orders in large specimens are almost twice as wide 
as long. In young specimens all the brachials are proportionally longer, and 
the bifurcations extend to nearly the full length of the rays; while in the 
adult they rise to only half way up. Arms biserial from the second plate 
after the axillary, short for the size of the calyx, and all rise to the same 
height. They are composed of moderately long joints, and each one has 
a well defined lateral projection for the support of a pinnule. 
Form of the ventral disk unknown; we observed, however, from a 
_ crushed specimen that it is composed of convex pieces apparently of a simi- 
lar arrangement to that of P. Hath, except that the ambulacral pieces are 
more projecting and comparatively smaller. The large interambulacral plate 
of the first row at the anal side is somewhat bulging outward, and wider than 
long. Column elliptic and twisted, the long diameter of the joints being at 
25 cm. from the calyx twice as long as the short one. The joints increase 
gradually downward to three times the length of those at the top, and near 
the base of the stem give off from their sharp ends two rows of small cirri, 
which follow the twist. 
Horizon and Locality. — Upper Burlington limestone ; Burlington, Iowa. 
Type in the Museum of Comparative Zodlogy. 
ftemarks. —In the branching of the arms this species approaches the 
genus Hucladocrinus, but while in the latter the successive orders of brachials 
form tubular appendages of the calyx closed from all sides, in P. incomptus 
they retain the characteristics of arm plates, being provided with open 
furrows. ‘The type specimen shows the bifurcations only to the third axil- 
laries above the costals, and it was supposed the species had but four arms 
to each side of the ray; while in fact it has from seven to ten, or as many 
as twenty arms to the full ray in some specimens. 
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