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HEX ACRINIDZ. 745 
fs | 
anal plate is generally of the same size as the radials, but obtusely angular 
at the lower face, and broadly truncate at the upper. Costals one or two; 
very small, in most cases not filling the whole width of the facet. When | 
two costals are represented, the plates are closely united, and form a syzygy. | 
Arms, so far as known, consisting of ten stout main trunks, with armlets | 
from one or both sides at intervals. Main arms as well as armlets composed | 
of quadrangular, single joints, and the plates of both pinnule-bearing, except 
the axillary ones. The pinnules are given off, so far as observed, from 
one side of the arms only, not alternately from opposite sides. 
The first interradial row of plates consists of one or three pieces, which | | 
are in contact with the lower brachials, and occupy the peripheral portions of | | 
the calyx. They are followed by inter-ambulacrals which enclose the orals. . 
Ventral disk from low-convex to hemispherical; the plates more or less 
nodose. Orals generally well defined; the posterior one largest, almost 
central, and pushed in between the other four. The plates covering the 
food-grooves consist either of a few large plates, or of two rows of small 
pieces, alternately arranged. 
Column round; axial canal small and circular. 
Distribution. — Hexacrinus is restricted to the Devonian. From America 
only two species are known, both from the Hamilton group, and of these 
but one or two specimens. have been obtained; while in Europe the genus is 
represented by many species, and specimens are quite abundant. 
Type of the genus: Hexacrinus melo Austin. 
fiemarks. — It is worthy of note that the two American species of Hexa- 
crinus have two costals, while the European, so far as known without excep- 
tion, have but one. They agree, however, in other respects so closely that 
we doubt the propriety of making this a generic distinction, especially as the 
two plates obviously form a syzygy, and take the place of one. 
Hexacrinus occidentalis W. and Sp. (nov. spec.). 
Plate LXXVIII. Fig. 10. : 
A small species. Dorsal cup higher than wide, broadly truncate at the 
base, very gradually spreading to the arm~bases; the sides a little convex; 
the plates moderately thick and without ornamentation; the suture lines | 
indistinct. , a 
94 | f 
