IIETEROCRINIDAE 
log 
5 to 7 ramiiles given off at intervals of 2, 3 or 4 brachials, is thoroughly shown, 
and the fourth arm, beginning the process of atrophy by which it is eventually 
eliminated, can be seen and studied. The area which it occupies is about one 
half that of the permanent lateral arms. The median arm is long and slender, 
with 17 to 20 short brachials, either ending beyond these in a narrow point or 
in an equal bifurcation of very slender finials. The cup is but little constricted, 
with height and width about the same. Surface in well preserved specimens 
somewhat punctate. 
H orison and locality. Trenton (Curdsville), Ordovician; Ottawa and Kirkfield, Canada, 
and Woodford County, Kentucky. This horizon is somewhat higher than that of C. punctatus. 
Cremacrinus kentuckiensis (Miller and Gurley) 
Plate 28. figs. 14, 14a, 15 
Calceocrinus kentuckiensis Miller and Gurley, Bull. 5, Illinois State Museum, 1894, p. 29, pi. 2, figs. 24, 25. 
This species, of somewhat coarser construction than C. articalosiis, is in- 
troduced with two specimens from the Trenton of Kentucky on account of the 
striking manner in which they illustrate the peculiar relation in this genus of 
the stem and anal tube. In figure 15 the tube occupies the middle space between 
the two arms at the posterior side, whieh in erinoids generally would be in the 
plane of bilateral symmetry. But that is not the case here, because the arm that 
is seen lying to the right of it is the fourth, or odd, arm, thus giving two arms 
on that side as against only one on the other. This would result in a complete 
loss of balance between the respective sides — a condition that is contrary to the 
whole tendency of the echinoderm organization — were it not that the equilibrium 
is preserved by the location of the stem in the plane of imperfect bilateral sym- 
metry passing through it and the median arm, leaving the anal tube in contact 
with the left posterior arm as it was before ; thus these two form a counterpoise 
to the two arms at the right, and the stem, instead of the anal tube, occupies 
the true middle position with regard to the calyx as a whole, as shown in 
figure 14a. When the odd arm atrophies in the next stage of the family, there 
must be a different arrangement in order to maintain a balance between the 
two sides, which have now become equal, with one large simple radial at each 
side. This is effected by the shifting of the stem into the same plane with the anal 
tube, thus establishing the perfect bilateral symmetry characteristic of the suc- 
ceeding g'enera. 
The r. ant. arm in both specimens is markedly smaller than the other lateral arms, as may 
be seen in figure 15, where one ramus of each arm adjoining the anal tube is visible on either 
side and their relative sizes may be compared. The median arm in this species is relatively 
shorter and stouter than in C. articulosiis, and bifurcates on about the 14th brachial into 
fairly strong branches. 
Horizon and locality. Trenton (Curdsville), Ordovician; near High Bridge, Mercer 
County, Kentucky. 
