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 grauulce. 



This encrinite is nearly allied to Platycrinus granulatus (Miller), hut it can he 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 Miclbeiini, Productus punctatus, and other carboniferous forms. 



EXPLANATION OF FIGURES. 



Fig. 1, a. View of the base of the calyx. 

 " b. Side view. 



POTERIOCRINUS. Miller. 



POTERIOCRINUS RIXOMBIFEKTJS. (N. S.) 

 (Tab. V. b, 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 pu ces 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 arc 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. 



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



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



POTERIOCRINUS TUMIDUS. (N. S.) 

 (Tab. V. B, 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 



