4.06 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, 7. 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 alternicirrus, which resembles them 
only in its alternicirration. 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 of joint-face. Assuming the simply 
radiate Entrochus plan to be the oldest, then the [socrinus 
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 Salanocrinus. 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 Jsocrinus 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 Jsocrinus 
is in the direction of stellation—the cutting-out of stereom 
that lends no strength to the column (ef. 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. Briinnich Nielsen (1913, Danmarks geol. Undersd¢g., 
il. Raekke, Nr, 26, pp. 6-8 & 81) rejects the genus Balano- 
crinus, because in a single species (P. paucicirrhus) he finds 
jOint-faces of both Isocrinus and Balanocrinus plan, with all 
