No. 189.] 53 



This species resembles the A. eucharis and A. calypso of the Hamilton group; 

 differing from the first in the number of arms, and in having the subdivisions of the 

 ray beginning as it becomes free from the cup; from the latter it differs in the greater 

 number of arms from the ray, as well as in the angular character of the supraradial 

 plates. 



ACTINOCRINUS HELICE ( n. s.). C/^^i 



Body short, broadly turbinate, the base overhanging the column. 

 Basal plates somewhat deeply notched at the suture-lines. First 

 radial plates proportionally large : second radials minute, qua- 

 drangular ; third radials slightly larger than the second and 

 pentangular, supporting supraradial plates on the upper sloping 

 sides, giving origin to two arms in the anterior ray, three in 

 the antero lateral rays and four in the postero-lateral rays, 

 giving the following formula : 



o 



Tr~Tj z=z 16 arms. 



Interradial areas with the first plate large, and one or two 

 plates above it. First anal plate large, heptagonal, round and 

 nodiform in the middle, and supporting three plates in the 

 second range, with two or three small ones above. 



Arms stout and of moderate length, composed of a double series 

 of plates, the centres of which are elevated, producing trans- 

 verse ridges on the arms. In the upper part of the arms, these 

 ridges are broken into nodes which are sometimes sharply 

 elevated. 



Surface of plates strongly nodose, becoming subangular or ridged 

 towards the margins. 



Column comparatively strong, composed of very unequal thick 

 joints ; the larger ones being angular on the periphery, and 

 sometimes showing a disposition to produce small nodes. 



This species is of the type of A. unicornis of the Burlington limestone; and it is 

 nearly related to A. prcRCursor of the Hamilton group, but that one has a stronger 

 body and more robust arms, with but three arms from each ray. It may be com- 

 pared with A. caulicidas of the Hamilton group, which has a smaller and less 

 spreading base, with more numerous and very slender arms. 



Associated with numerous well-marked specimens of this species are several in- 

 dividuals which have either an irregular arrangement of the arms, or, when regular- 

 ly arranged, have a smaller number of arms : these have always two arms in the 

 anterior ray, and, with two exceptions, four arms in the postero-lateral rays. (These 

 two have a formula of 



'gzE =: 12 arms.) 



In other respects they are all so precisely similar, that no distinction can be made, 

 unless the specimens show the entire formula. 

 The specimens with regular arm-formula of 



2-5- =Z 12. 

 US 



may be designated as a variety = Actinocrinus helice, var. eris. 



