HEXACRINID^. 745 



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 i-epresented, 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 iirst interradial row of plates consists of one or three jjieces, 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. 

 2'i/pe of the genus : Hexacrinus melo Austin. 



Remarks. — 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, liowever, 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 L XXVIII. Fig. 10. 



A small species. Dorsal cup higher than wide, broadly truncate at the 

 base, very gradually spreading to tlie arm bases ; the sides a little convex ; 

 the plates moderately thick and without ornamentation; the suture lines 

 indistinct. 



94 



