IIETEROCRINIDAE 
125 
crowns in the material now in hand, considerably more than half of them have the decided 
appearance of a tightly strung bow, and practically all of these have the stem lying close under- 
neath on the side opposite the curve. No crinoid with such a curvature of the crown could 
conceivably occupy a habitual position either erect, or hanging down along an erect stem ; 
whereas the form here exhibited lends much plausibility to the idea of a recumbent habitus. 
A single specimen, figure 15, already alluded to in the discussion, shows the crown almost 
fully opened upon the hinged articulation with the base, but this is believed to be incidental, 
due to some spasmodic movement at death. 
It also illustrates the great and final extension of the two large radials toward the tube, 
they being now separated only by the inferradials of the vanished arms, which are here re- 
duced to a relatively small size, and with which, indeed, they may be occasionally fused. 
Along with this, the visible anal plates are much reduced, being but little exposed at the base 
of the tube, in strong contrast tO' their condition as usually seen in the Ordovician, Silurian 
and Devonian species; nevertheless the tube extends to the full height of the arms (fig. i6a). 
Secondly, the species exemplifies in a striking manner the variations that must be ex- 
pected and allowed for in a prolific form, especially one that is marked by such strong charac- 
ters that it is readily recognized, even in many cases from fragments. As above stated, there 
are about forty good specimens in the collection with the crown intact, all from a single re- 
stricted colony in the Keokuk group. They range in size from 70 mm. height of crown down 
to 20 mm., so that we have the opportunity to compare the condition of young and adult. Upon 
tabulating the characters as exhibited by this series of specimens, it is found that as to several 
of them there is a definite progressive variation according to the maturity of the individual : 
The proportions of the cup. Typically as seen from the left anterior radius this be- 
comes wider toward the hinge-line, where its width exceeds the height from there to the top 
of the superradial. A composite of five mature specimens gives 15 mm, height by 16 mm. 
width, with a maximum of 17 mm. by 20; whereas in the young of about 20 mm. height of 
crown these proportions are reversed to 8 mm. height by 5 mm. width (pi. 30, fig. 20). 
The bifurcation of the median arm. Here this reaches its culmination for the family to 
a maximum of 8 and 10 branches. It bifurcates throughout the species on "from the fifth to 
the tenth brachial — in 70 per cent on the seventh or eighth, which is the normal — but the 
number of branches differs with growth. Specimens of from 35 to 70 mm. height of crown 
have from 6 to 8 (rarely 10) finials, while those of 20 to 35 mm. have from 2 to 4; in the 
youngest individuals only about 20 mm. high, where the median arm narrows almost to a 
point, there is evidence of a single bifurcation upon the sixth or seventh brachial. 
In like manner the number of axil-arms increases with age, specimens above about 
40 mm. in height having 5 and 6 on a side, the former number predominating ; those from 
30 to 40 mm. have usually 4, and below 30 mm. 3. The juvenile condition in this and the 
preceding species, with their small number of lateral arm-branches, recalls the arm-structure 
of En-cheirocrinus, from which the axil-arm system may have been derived. In the present 
species the Alphabrachs and Betabrachs range in their series from 3 to 5, the prevailing num- 
ber being 4. 
From this resume of the facts resulting from a careful tabulation, it is evident that most 
of these variations are progressive with individual growth, and that, in this form at least, 
such differences would not form any sure basis for the separation of species. 
The progressive variations thus exhibited due to growth in the individual are analogous 
to those which occurred in the same characters during the life of the family from the earlier 
to the later geological stages. 
Horison and locality. Keokuk group, Mississippian ; Indian Creek, Montgomery County, 
Indiana; and rare in the vicinity of Keokuk, Iowa. 
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