498 FRANK SPRINGER 



Lecanocrinus (Plate VI, Fig. 13), and Anisocriniis (Plate VI, Fig. 10), 

 in which the posterior rays arch over the anal plate, leaving no 

 vacant space above it. In Calpiocrinus we see its simplest extension 

 by the addition of single plates in succession above it (Plate VII, 

 Fig. 7). In Temnocrinus we have a further modification by the 

 addition of two or three plates abutting on the anal (Plate VII, Fig. 15). 

 The development was rapid, for in another Silurian genus, Sageno- 

 crinus, in which the dextrorse asymmetry caused by the presence of 

 a radianal is still a feature, we have a perfect example of this plan 

 in its fullest extension, where the anal area, following upon an angular 

 posterior basal, is completely filled to the height of several orders 

 of brachials, with solid plates, forming a regular continuation of the 

 calyx wall. Because of its remarkable development in that genus, 

 this form of anal structure may appropriately be called the Sageno- 

 crinus plan. It is carried forward with equal perfection into the 

 Carboniferous by the genus Forbesiocrinus. in which the radianal has 

 been eliminated ; and it ends in the Keokuk and Warsaw with species 

 in which the asymmetry of Taxocrinus forms without a radianal has 

 nearly disappeared, and the bilateral symmetry of the calyx is almost 

 perfect. In most cases, however, asymmetry may be seen in the 

 longer slope of the right shoulder of the posterior basal (Plate VII, 

 Fig. 20). Where the posterior basal is truncate, the succeeding plate 

 may be taken as the anal x, but where it is angular, as is most fre- 

 quently the case, the anal x is apparently spht in two, giving "rise to 

 two or more series of other plates above. While there are thus a 

 few cases, as indeed we find also in the Silurian in forms like Calpio- 

 crinus, which seem to be bilaterally symmetric, yet it is a fact that 

 the dextrorse aysmmetry remains also as a general characteristic of 

 this form as well as the Taxocrinus form. If the posterior basal is 

 angular above, the right sloping face is generally the longest, and 

 the series of plates following it a little the largest; if it is truncate 

 and followed by one large plate which is angular above, the same 

 remark may apply to the second plate ; if it is truncate with sloping 

 shoulders, the series from the right shoulder usually seems a little 

 the stronger, and rises higher along the ray than the others. 



Between such genera as Calpiocrinus, Temnocrinus, and Sageno- 

 crimis there are considerable variations of this plan; but throughout 



