Terms in Crinoid Morphology. 65 



])apcr, that tlic anal a:" descended into the cup from above ; and 

 I believe tliat the lowest, median, posterior plate of the ventral 

 tube is always this same plate, whether it be right above the 

 radials, as in locrinus and Mei-ocrtnus, resting on the radials, 

 as in JJeterocri7ius and Castocrinus^ between the radials but 

 not in line with them, as in Homocrinus and Bendrocrinus, 

 in line with the radials, as in Jiotryocrinus and CyathocrinuSj 

 or rising above the radials again, as in the later Decado- 

 crinidaj and in the larval Antedon. In this view I have the 

 misfortune to differ from Messrs. Wachsmuth and Springer, 

 who apply the term " anal x " to the lowest plate of the tube 

 only when it is partly or entirely within the limits of the 

 dorsal cup. They " apjdy the term ' anal plate ' only to those 

 taking part in the dorsal cup. All others are plates of the 

 anal tube or the ventral sac." That this is not, in their 

 opinion, a mere difference of terminology is shown by the 

 arguments that they have based on this supposed difference. 

 If, however, we consider such a form as Bolryocrinus^ we 

 shall see that the anal x is of exactly the same shape and 

 provided with the same axial ridges as the plate that rests 

 immediately on it : there is no visible difference between 

 them, and whatever the one is that we should suppose the 

 other must naturally be. It is merely for convenience, and to 

 distinguish it from the other plates of the cup, that we call 

 one of them " anal x^ 



In exactly the same way, the third anal plate that enters to 

 a greater or less extent into the dorsal cup in such genera as 

 Dendrocriiius^ Poterioo'inus, and Decadocrinus would appear 

 to be merely the small plate that in Botryocrinus, Cyatho' 

 crinus, and such forms, is seen on the right of anal x^ resting 

 partly on it and partly on the right posterior radial, and 

 corresponding to a similar plate on the left. In other words, 

 calling these two plates rt and It respectively, rt is outside the 

 cup in Cyathocrinus but partly inside it in Pater iocrinus. 

 Messrs. Wachsmuth and Springer, however, in their paper 

 on the Perisomic Plates (p. 385) have advanced the view, 

 apparently for the first time, that in the Poteriocrinites " a 

 new plate was introduced beneath the other, a sort of third 

 anal." That is to say, in their opinion the third anal of 

 Poter iocrinus is a fresh development without any homologue 

 in the CyathocrinidEe and Botryocrinites. But surely the 

 constancy in shape and position of the anal cup-plates x and 

 rt is hardly consistent with the idea that they are merely 

 supplementary plates developed to suit the needs of those 

 particular genera in which they appear. It seems more in 

 accordance with the principles that have hitherto governed 



Ann. (S) Mag. N. Hist. Ser. 6. Vol. ix. 5 



