FROM THE CARBONIFEROUS LIMESTONE OF IOWA. 595 
the inferior angles, and terminates at the base of the arms, marking out three triangular spaces on each 
piece, which are also studded with granule. 
This encrinite is nearly allied to Platycrinus granulatus (Miller), but it can be readily distinguished 
from that species by its more depressed form, smaller size, and flattened basal plate. 
Formation and locality.—It occurs in the quarries near Burlington, Iowa, in the encrinital beds of 
the carboniferous limestone, just above the oolitic members of this formation. It is associated with 
Spirifer striatus, Orthis Michelini, Productus punctatus, and other carboniferous forms. 
EXPLANATION OF FIGURES. 
Fig. 1, a. View of the base of the calyx. 
«¢ 8. Side view. 
POTERIOCRINUS. Miller. 
POTERIOCRINUS RHOMBIFERUS. (N. S.) 
(Tab. V. 3, fig. 2, a, b, c.) 
Calyx.—Conical, plates massive, surface garnished with heavy ridges. Basal plate pentagonal, small, 
scarcely a line in diameter, entirely concealed from view when the supra-columnar joint remains 
attached. 
First series of pieces five, pentagonal, or irregularly hexagonal, forming by their union a shallow cup, 
with a decagonal margin, in which five of the angles are salient, and five re-entering. Second series five, 
three hexagonal and two heptagonal, resting by their inferior salient angles in the retiring angles of the 
plates beneath, and supporting on their oblique superior edges, the five superior or arm-bearing plates, 
and two accessory pieces. The superior pieces are larger and more massive than the plates beneath ; 
their width is about one-third greater than their length, and each presents a large bevelled surface at 
the superior margin for the articulation of the arms. This surface is moderately excavated, and occupies 
about four-fifths of the entire width of the plate. Accessory pieces two in number: one small and quad- 
rangular, rests in a retreating angle between the heptagonal plates of the second series, the other irregu- 
larly pentagonal, is wedged in between two of the arm-bearing pieces. 
The surface of the calyx of this species is marked by heavy ridges, five of which commence on the 
first series of plates, and crossing the sutures, bifurcate on the second series; the branches then proceed 
to the superior pieces, where they again unite with each other, in such a manner as to form a series of 
five lozenge-shaped figures around the body, enclosing as many shallow depressions. 
Column, arms, and capital integument unknown. 
It occurs with Platycrinus discoideus, Pentremites melo, P. stelliformis, and other fossil remains in the 
encrinital beds of the carboniferous limestone at Burlington, Iowa. 
For the fine specimen figured we are indebted to Dr. Matthews, of Burlington, who obligingly furnished 
us with a number of interesting Crinoids from his choice collection. We have not observed it elsewhere 
in the Northwest. 
EXPLANATION OF FIGURES. 
Fig. 2, a. Basal view. : 
«$6, Side view, exhibiting the two accessory pieces. 
«* ¢, View of the side, showing the form of the superior plates. 
POTERIOCRINUS TUMIDUS. (N. 8.) 
(Tab. V. B, fig. 3, a, 5.) 
i i he summit, tumid in the middle, plates 
Calyx.—Subovate, rapidly enlarging from a narrow base to t ; » pla 
ae, slightly Swnivet, moderately thick, column round, very slender. Basal plate concealed. First 
series of plates small, pentagonal, closely adhering among themselves, forming when united a little cup ; 
its superior edge with five obtusely salient, and five very slightly re-entering angles. Second series five, 
three hexagonal and two heptagonal, their height nearly four times that of the first series, bent a little 
