320 Mr. F. A. Bather on British Fossil Crinoids. 



plexing that it will be well to give a short historical sketch 

 of the views of the leading authorities. 



In 1879 Messrs. Wachsmuth and Springer (Rev. I. (71-72) 

 and footnote, Proc. pp. 294, 295) noting that Hall had wrongly 

 described the ventral sac of locrinus polyxo as an arm, wrote : 

 " the similarity in the appearance of the ventral sac and the 

 aims and pinnule is indeed most striking. If there is in 

 nature any such thing as a transmutation of one organ into 

 another, it would seem that such was the case here, and this 

 may lead to a better understanding of the functions of the 

 ventral sac " ; and again (Rev. I. (65), Proc. 187c), p. 288) 

 " was not the ventral tube here [in locrinus'], and in Crinoids 

 generally, originally a modified arm ? This, if true, would 

 at once explain why the anal area leans always towards the 

 right and never to the left side of the body." locrinus 

 (PI. XIV. fig. 5) then was taken by them as the starting-point : 

 the axillary plate that supports on the left the ventral sac 

 and on the right an arm, was regarded as a costal with inter- 

 radial functions, and the plate on which it rests as a radial 

 homologous with the other radials of the dorsal cup. In 

 Dendrocrinus (PI. XIV. fig. 15) the right posterior radial was 

 supposed to have split transversely into two plates (R and 

 R + ), both "strictly radial" and "homologous with the 

 single radial in other Cyathocrinidas." The plate ( x ) that 

 in locrinus was supported by a costal, has here passed down 

 into the dorsal cup and is " a regular anal plate." In Homo- 

 crinus \J*\. XLV. fig 16) " the suture between the sections of 

 the compound plate (R and R + ) is sloping instead of hori- 

 zontal " and "by this trifling alteration" R-t- is "trans- 

 formed into an anal plate." " This was the first step towards 

 a Poteriocrinus anal arrangement, and in fact to complete it 

 required only the interposition of a third small plate," i. e. 

 a plate (t) of the ventral tube, to come down between x and R 

 (PI. XIV. fig. 26) . Cyathocrinus (PI. XIV. fig. 20) arose from 

 Dendrocrinus by "the consolidation of the compound plate 

 into one," i. e. R+ is rejoined to R, but occasionally remains 

 as in Botryocrinus and Barycrinus (PI. XIV. fig. 30). Simi- 

 larly in lJeterocrinus (PI. XIV. fig. 6), which then included 

 Ectenocrinus (PI. XIV. fig. 7), Wachsmuth and Springer re- 

 garded R+ as the lower part of a compound radial; the two 

 plates that occur in the right posterior and right and left 

 anterior radii were considered homologous with a complete 

 single radial. So far, whether correct or not, they were con- 

 sistent : but in Hybocrinus (PI. XIV. fig. 3) they regarded the 

 large plate, which exactly corresponds in position to R-f- of 

 Homocrinus } as an anal plate pure and simple homologous 



