Il4 PROCEEDINGS OJ" THE ACADEMV OF [1886. 



that unites radials and brachials, extends to all bifurcating 

 plates of the Poteriocrinidap. In Urisocrinus, Eiipachycrinus 

 and Graphiocrmus, species with ten arms, it is found also among 

 the proximal arm plates, not, however, in the allied Scytalocrinus 

 and Decadocrinus, in which all arm plates above the brachials 

 are united by suture or elastic ligament. The same was prob- 

 ably the case in species with branching arms, in which all inter- 

 mediate joints between the axillaries are without either ridges or 

 fossae. A sutural union also connects the brachials in cases 

 where they consist of more than one plate, the apposed faces 

 showing no traces of a syzygy or fossae. In the arms of the 

 Cyathocrinidse there are, so far as observed, no articular ridges 

 nor axial canals, and no ligamentous fossfe, not even between 

 radials and brachials, nor upon the axillaries. The apposed 

 faces of all their joints fit closely together, the distal end being 

 slightly concave, the proximal to the same degree convex, so 

 that we may assert that their mode of union has been either by 

 suture or of a somewhat similar nature, and that the arms were 

 either immovable or their motions limited, and probably of a 

 mere passive character. This difference in the articulation of tlie 

 two types was noticed by J. S. Miller when describing the t3^pical 

 genera, for he placed Cyathocrinus separately among Inarticulata, 

 and Foteriocrinus among Semiarticulata. These names, as 

 applied to the rays, are very characteristic of the two groups, 

 for the Poteriocrinidae are in their articulation much more highly 

 differentiated, approaching in some cases the Neocrinoidea, 

 Miller's Articulata. 



In none of the Poteriocrinidtie has the ventral covering, with 

 the exception of the so-called ventral sac, ever been observed, 

 while that of the Cyathocrinidae is comparatively well known. 

 This, however, is parti}' explained by the condition of the radials, 

 which in the Cyathocrinidie had ample space for the reception of 

 interradials, contrary to the Poteriocrinidae, in which the articu- 

 lation extends over the whole width of the radials, and the inter- 

 radials, partly or wholly, had to rest against the muscle plates as 

 in the Symbathocrinidae. In the Cyathocrinidae the interradials 

 were persistent through life, but they may have become resorbed 

 in the Poteriocrinidae before reaching maturity. The condition 

 of the ventral side among the Cyathocrinidae varies considerably, 

 and even among the species now referred to the genus Cyatho- 



