90 OWEN AND SHUMARD'S FOSSIL CRINOIDEA. 



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 sup- 

 porting 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 

 arras. 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 quadrangular, 

 rests in a retreating angle between the heptagonal plates of the second series, the 

 other irregularly 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 lozenged-shaped 

 figures around the body, enclosing as many shallow depressions. 



Column, arms, and capital integument unknown. 



It occurs with Platijcrinus 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. Mathews, of Burlington, 

 who obligingly furnished us with a number of interesting crinoids from his choice 

 collection. We have not observed it elsewhere in the North West. 



EXPLANATION OF FIGURES. 



Fig. 2. a. Basal view. 



" h. Side view, exhibiting the two accessory pieces. 



" c. View of the side, showing the form of the superior plates. 



POTERIOCRINUS TUMIDUS. (New sp.) 

 PI. XI. fig. 3, a, b. 



Calyx. — Subovate, rapidly enlarging from a narrow base to the summit, tumid in 

 the middle, plates smooth, slightly convex, 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 inwards superiorly; inferior angles slightly salient. Superior 



