188G.] NATURAL SCIENCES OP PHILADELPHIA. 131 



STENOCRINUS, nov. gen. 

 (Syn. Heterocrinus in part). 

 {arci'6rT, nari'ow ; Kpivov, a lily). 



Generic Diagnosis. General form subcylindrical ; cal3x small; 

 arms long. 



Basals five ; of a more regular form than in Heterocrinus, 

 angular at their superior margins. They form a cup, which 

 spreads but little from the column upwards. 



Radials irregular, consisting of one or two plates ; two of the 

 rays sometimes three being composed of two segments each, 

 the others of only one. The single plates are almost as large as 

 any of the compound ones. Only four of the radials supported 

 upon the basals ; the fifth one smaller, resting upon the truncate 

 upper side of the azj-gous plate, and occupying toward the latter 

 a similar position as the two sections of the compound radials 

 toward each other. The smaller radial is transversely penta- 

 gonal, and resembles in outline a bifurcating plate. It supports 

 upon its sloping right side the brachials ; toward the left the 

 ventral sac as in locrinus. Brachials generally four to each ray, 

 the upper one axillary, and supporting the two main arms, which 

 sometimes branch again, and give off at regular intervals, from 

 alternate sides, strong, arm-like pinnules. All arm joints which 

 have no pinnules are strictly quadrangular, while the pinnule- 

 bearing joints have almost the form of bifurcating plates. 



Axygous plate pentagonal, shorter than adjoining radials. 

 Ventral tube, near the base composed of qnadrangular, heavy 

 plates, generally longer and narrower than the brachials. The 

 structure of the tube in its upper portions not known. The plates 

 of the ventral side have been observed only in H.juvenis, in which 

 they are composed of five comparatively larga interradial pieces, 

 enclosing a small oral plate as in Haplocrinus. Column large, 

 pentagonal, pentapartite, the sutures directed interradially, giving 

 to the segments a radial position. Axial canal large, pentagonal ; 

 angles interradial.^ 



Geological Position^ etc. Stenocrinus is known only from the 

 Silurian of America. 



We refer to Stenocrinus the following species, all previously 



1 The columnar canal of Stenocrinus is correctly represented in fig. 12 of 

 PI. vi, not, however, in fig. 13, in wliich the rays should be directed inter- 

 radially. 



