318 Mr. F. A. Bather on British Fossil Cnnoids. 



paper was published before Mr. Bury's researches on Antedon ; 

 had he been acquainted with these, and also better acquainted 

 with the earlier Palaeozoic cnnoids, he would hardly have 

 written as he did. 



On the other hand, the tendency of infrabasals to diminish 

 in size during geological time, in other words the general 

 change of Dicyclica into Pseudomonocyclica, suggests that 

 the Dicyclic type is more primitive than the Monocyclic. In 

 this connexion we may recall Wachsmuth's statement that 

 basals and infrabasals seem to be early developed in the 

 individual, " for they are as large in the young as in the 

 adult, and do not show much increase in proportions in later 

 geological epochs ' ? (Rev. I. (19), Proc. 1879, p. 242). One 

 might also allude to the stems of some Bohemian Cystidea, 

 which are composed of alternating circlets of plates as though 

 developed in extensions of the calycal perisome. The only 

 objection to the derivation of Monocyclica, through Pseudo- 

 monocyclic stages, from Dicyclica, lies in the involved change 

 of orientation. Here, however, it seems to present less diffi- 

 culty than on the converse hypothesis. The atrophy of 

 infrabasals is we see a very gradual process, and, as proved 

 by specimens of Forbesierinus in the British Museum, it does 

 actually appear to be in some cases accompanied by a change 

 in the position of the lobes of the axial canal. 



Bo far as the Fistulata are concerned there is no geological 

 evidence to show whether Monocyclica, Pseudomonocyclica, 

 or Dicyclica be the older. All my contention is that Mono- 

 cyclic forms may, should other evidence render it probable, 

 be derived from Dicyclic. 



The distinction between an infrabasal ring of 5 plates and 

 one of 3, is of far inferior importance. It is acknowledged that 

 3 infrabasals represent the original 5, of which two pair have 

 fused and the 5th remains as a rule in its pristine condition. 

 In the Fistulata, as in Antedon, this unaltered infrabasal is 

 that in the anterior radius ; the fused pairs are therefore those 

 of the left and right sides respectively. This fusion of infra- 

 basals may be regarded as a generic character, but nothing 

 more; for any genus with 5 infrabasals may have its ana- 

 logue with 3 infrabasals, as, for example, Cyathocrinus has 

 Gissocrinus. It is fairly obvious that an infrabasal ring of 

 5 plates is more ancestral than one of 3, and that one small 

 and two large infrabasals represent a more archaic stage than 

 do three equal infrabasals. In Stemmatocrinus the infrabasals 

 are anchylosed into a pentagonal disk, and this is no doubt a 

 later development. No cases of 4 or 2 infrabasals are known 

 in the Fistulata. 



