94 
SMITHSONIAN INSTITUTION 
ramules, seems a long jump. But the intermediate stage is beautifully shown in the Gotland 
species, Calceocrinus gotlandiciis, where a sternward diminishing series of rami similar to 
those of Form A is combined with a succession of unequal-faced axillaries following an 
unsymmetric primibrach. The three stages are shown in the accompanying text-figures. 
If in b the left ramule of each ramus enlarges, and all parts to the right are dwarfed and 
hidden behind it, as in certain species of Form C, the arm system of c will result. 
The prime morphological factor at the bottom of these modifications is the change in 
shape of the lateral axillary primibrach (lAx) from equal- to unequal-faced, from which 
the unequal division of the ray necessarily follows. This actually begins with Form B, in the 
typical species of which the articulating faces of the primibrach differ in length about as 
4 to 3, giving rise to an unsymmetric division witbin the ray. Therefore we have in this a 
third broad character by which Form A is differentiated from all the rest of the family. 
u 
c. 
Fig. 2. Three Stages in Development of Axil-Arm 
a. Cremacrinus tiihidiferiis. Equal rami from symmetric IBr. 
b. Calceocrinus gotlandiciis. Unequal rami from unsymmetric IBr. 
c. Calceocrinus foerstei. Axil-arm system, with only parallel ramules visible from short faces 
of IBr and successive axillaries. 
Other Progressive Characters 
Thus the axil-arm as a post-Ordovician development is a most conspicuous character 
marking the evolutionary succession of the family in geological time. It is now a further 
striking fact that there is correlated with it another character more often seen because it ap- 
pears in the calyx plates, which are the most frequently preserved in the fossils. 
That is the greater or less separation of the two large simple radials by the connection, 
or want of connection, of the upper and lower segments of the 1. ant. radial. The change was 
incident to the transverse extension of those radials to form the support of the axil-arms, so 
that the two structures underwent a parallel progression. In all the Ordovician species the 
segments were broadly connected, the inferradial being an elongate, quadrangular plate ; this 
was constant in Form A, of which some species passed over into the Silurian. In Form B 
of the Silurian the connection was maintained, with a tendency to modification by the narrow- 
ing of the inferradial at the top, leading toward an acute-angled apex. In the Silurian Form C 
the complete change was effected by the transformation of the inferradial from a quad- 
rangular to a triangular plate, and the consequent severing of connection between the two 
segments. Exceptionally as a survival the Ordovician type persists in this form, as in Calceo- 
criuus foerstei, C. interpres and C. pinmdatiis ; and rarely in other species the two segments 
may touch by their apices. Finally with Form D in the Devonian and Carboniferous the 
separation of the segments by the interposition of the simple radials was completely estab- 
lished, and became a constant character of the genus Halysiocrinus. 
