46 THE VOYAGE OF H.M.S. CHALLENGER. 



series consists of " a single row of plates, longitudinally arranged, the outer side rounded 

 and forming a prominent ridge, which gives the appearance of an arm." ^ 



The physiological condition of this type appears to me to be very similar to that of 

 Reteocrinus nealli, i.e., the anal interradius is supported by a special row of plates, the 

 lowest of which are more or less incorporated into the calyx, while the upper ones are 

 closely surrounded by perisomic plates. 



In Poteriocrinus and Eupachycrinus this anal appendage is practically reduced to one 

 plate, the so-called third anal plate, or the first plate of the ventral tube ; but it rests on 

 the first anal or azygous plate, which is itself supported by a basal. This azygous plate 

 is absent in Ceriocrinus, and the first plate of the tube (anal appendage) comes dowai to 

 rest directly upon a basal. 



Another curious modification is presented by locrinus. The posterior side of the 

 calj'x is occupied by a large plate that rests between two basals and is in line with the 

 radials. It has been variously described as a radial and as an azygous piece.^ On its 

 upper surface is a triangular plate which " supports on its right sloping side the usual 

 number of brachials, and on the left a row of quadi'angular plates, vertically arranged, 

 extending to the tips of the arms, and forming the posterior wall of a large ventral tube. 

 In external appearance these plates resemble the brachials and arm plates, only they are 

 somewhat higher and not quite as wide ; they are gibbous and form an elevated ridge, 

 which causes this appendage to resemble an arm or a branch of the ray." This median 

 ridge extends to the full length of the ventral sac, and it is bordered, just as is the case 

 in Reteocrinus nealli, liy a number of more delicate perisomic plates. I have little 

 doubt that it served the same purpose in both cases, supporting the anal interradius, 

 though in no way specially grooved for the reception of the hind gut. It is well figured 

 by Meek in the Palaeontology of Ohio, vol. i. pi. i. fig. 9b. 



1 Revision, part L p. 69. 



- For further information upon this subject, see Wachsrauth and Springer, Revision, part i. pp. 65, 71 ; and Ariie7: 

 Jovrn. Sci. and Arts, voL xxvi. 1883, pp. 370, 376 ; also P. H. Carpenter, Quart. Journ. Geol. Soc, vol. xx.wiii., 1832, 

 lip. 306, 307. 



