Mr. F. A. Bather on British Fossil Crinoids. 317 



family as pseudomonocyclic. The Heterocrinidaj, Anomalo- 

 crinidse, and Belemnocrinidaj are all truly monocyclic ; while 

 the remaining Fistulata are undoubtedly dicyclic. Is this 

 fact alone enough to warrant a separation into two groups ? 



This question involves two others. First ; can one group 

 be derived from the other? Second ; if so, which group is 

 ancestral ? Let me repeat that this is not a question of the 

 origin of Pseudomonocyclica, for they, it is obvious, are 

 derived from Dicyclica ; but it is a question of the relations 

 between Dicyclica and Monocyclica Vera. 



It has hitherto been generally supposed that the Mono- 

 cyclic stage is the older of the two. No particular reasons 

 have been given for this opinion beyond the natural one that 

 a dorsal cup formed of two circlets only is simpler than one 

 formed of three. Such an argument however, unless sup- 

 ported by Palaeontology or Embryology, really begs the 

 question ; the hoof of Equus caballus is from one point of 

 view simpler than the 5-toed foot of Phenacodus, but it is 

 not simpler when origins are considered. Those who take 

 the Monocyclic type to be the older may suppose that infra- 

 basals were subsequently developed as a new and sudden 

 accession to the elements of the dorsal cup. This idea again 

 has the merit of simplicity, but it overlooks the difficulty of a 

 change in the orientation of the stem. It is true that in the 

 species of Antedon described by Mr. Bury the infrabasals 

 appear after the basals ; but Antedon is so specialized a form, 

 and the infrabasals are in so extremely degenerate a state, 

 that no morphologist could attach any importance to this fact. 

 Should the monocyclic base of the Inadunata Larviformia, 

 which Wachsmuth and Springer regard as more ancestral 

 than the Fistulata, be adduced in favour of this view, it would 

 be enough to point out that, with the exception of the very 

 irregular Pisocrinus ( Wenlock and Niagara), they have not 

 been found below the Devonian. Dr. J. Walther*, who 

 likewise derives Dicyclica from Monocyclica, is forced by diffi- 

 culties of orientation to homologize Monocyclic basals with 

 Dicyclic infrabasals, and Monocyclic radials with Dicyclic 

 basals, while he regards the radials of Dicyclica as an entirely 

 new element in the dorsal cup. This view implies that the 

 arms, the anus, and the elements of the tegmen have turned 

 through an angle of 36° : a comparison of focrinus and Mero~ 

 crinus (Plate XIV. figs. 5 & 11) shows the extreme difficulty 

 of accepting such a reversal of our accepted homologies. It is 

 just to Dr. Walther to remember that his very suggestive 



* " Untersuchungen iiber den Bau der Crinoiden u.s.w.," Palaeonto- 

 graphica, xxxii. pp. 155-200. Stuttgart, 1886. 



