114 SMITHSONIAN INSTITUTION 



the wall is weakened by the interposition of an anal tube at the posterior side. 

 There are exceptions and intergradations in regard to this feature, as in all 

 such cases; and it is difficult to frame a definition fixing an exact limit which 

 will hold good everywhere. I have drawn the line at the first plate of the anal 

 series above the posterior basal— the anal x as commonly designated. If this 

 plate is free from sutural attachment at least on one side, it cannot be said to 

 be incorporated in the calyx, and a form so constructed will not belong to 

 group A. On the other hand there will necessarily be some genera which upon 

 the criterion here given must be placed in group A, notwithstanding the fact 

 that they may have a very apparent anal tube which would seem to throw them 

 into group B. Such are intermediate forms like Synerocrimis and Meristo- 

 crinus, the systematic position of which must be decided upon a slight prepon- 

 derance of evidence among characters more or less contradictory. This group 

 also includes, in addition to those with a solidly filled anal side, such genera as 

 IchthyocrinuSj Metichthyocrinus, Wachsmuthicrinns and Nipterocrinus, in 

 which the posterior basal is not differentiated, and there are in the calyx 

 (except the inconspicuous radianal in Ichthyocrinus) no anal structures what- 

 ever. 



B. With a weak calyx: Taxocrinoidea. This will include all forms in 

 which the posterior basal is followed immediately by a tube-like series of plates 

 no part of which is fully incorporated in the calyx, but which is bordered on 

 one or both sides by perisome, and so to that extent separated from the adja- 

 cent brachials. In all genera composing this group, without exception, the 

 posterior interradius is differentiated from the others by the interposition of 

 an anal tube. The crown in genera of both groups may be weakened by the 

 presence of a pliant perisomic integument instead of suturally united inter- 

 brachials above the zone of the radials in all the interradii, but in this group 

 such perisomic integument is always to be found at the anal side between the 

 posterior radials and down to the basal, whatever the structure may be else- 

 where. Some exceptional intermediate forms in which this condition is closely 

 approached are placed in the first group upon the weight of other characters. 



The two divisions thus defined are substantially the same as those pro- 

 posed by me in 1906 (Jour. Geology, vol. 14, p. 513), and further investigation 

 has shown that the differences between them are even more striking and funda- 

 mental than was before supposed. Taking as typical of the two types of struc- 

 ture the genera Forbesiocrinus and Taxocrinus, which have been often 

 confounded in the literature : 



In Forbesiocrinus the posterior basal is angular or truncate above, and the 

 distal face is united to the succeeding plates by powerful ligamentous connec- 

 tions of the same type as that between the basals, radials and primibrachs 

 (PI. XXI, figs. 1, 2, 3). The succeeding plates of the anal side are similarly 



