406 On British Fossil Crinoids. 



Balanocrinus didactylus, but the joint-faces of the holotype 

 are admittedly obscure. The variety is in any case un- 

 necessary. 



There may be one or two more Tertiary species of the 

 same general type, i. e. alternicirrate Balanocrinus, and it 

 seems probable that all these species may be genetically 

 related. It is, however, improbable that they are related to 

 the recent Endoxocrinus alteimicirrus, which resembles them 

 only in its alteruicirration. What was the advantage of 

 this peculiarity, or how it first arose, are questions that 

 still seek an answer. 



Another question is as to the origin and advantage of the 

 Balanocrinus plan o£ joint-face. Assuming the simply 

 radiate Entrochus plan to be the oldest, then the Isocrinus 

 plan developed from this by the concentration of the liga- 

 ments in 5 pillars, and it is plain that this plan must have 

 preceded that of Balanocrinus. Possibly the reduction of 

 the radial ridge-groups merely continued when once started, 

 and was in part accelerated by their decreasing utility 

 according to the law of economy. 



Such an explanation suggests the further hypothesis that 

 the Balanocrinus plan arose from that of Isocrinus several 

 times over between the Trias and the Oligocene, and that 

 it does not characterize a homogeneous monophyletic 

 genus. 



The general tendency of economy of material in Isocrinus 

 is in the direction of stellation — the cutting-out of stereom 

 that lends no strength to the column (cf. Ionic as derived 

 from Doric, or Gothic from Norman). But in those species 

 that did not adopt this mode of retrenchment, remaining 

 cylindrical or basaltiform, the economy was effected in the 

 reduction of unnecessary ridges on the joint-face, possibly 

 combined with stronger radial ligaments. 



Finally, on this hypothesis, the alternicirrate Balanocrini 

 of the early Tertiary rocks form a homogeneous group, 

 derived from some Cretaceous species, such as the Upper 

 Senonian Pentacrinus bronni Hagenow, the Lower Danian 

 P. paucicirrhus Nielsen, and the Upper Danian P. crassus 

 Nielsen. These three form a continuous series of alterni- 

 cirrate forms, with joint-faces of Balanocrinus plan, and so 

 closely resemble one another in stem-characters that no differ- 

 ence is apparent in the descriptions or figures. 



Dr. Briiunich Nielsen (1913, Danmarks geol. Unders^g., 

 ii. Raekke, Nr. 26, pp. 6-8 & 81) rejects the genus Balano- 

 crinus, because in a single species {P. paucicirr/ius) he finds 

 joint-faces of both Isocrinus and Balanocrinus plan, with all 



