PALAEONTOLOGY OF IOWA. 607 



downwards about half the length of the radial plate, and 

 one-third the entire length of the body : each of these areas 

 contains about thirty-five pore pieces on each side. 



SURFACE finely striated longitudinally and transversely. 



The specimen figured is crushed, so that the character of the summit cannot be 

 be determined : the base also is apparently imperfect, and is a little shorter than 

 the distance to base of pseudo-ambulacral areas. 



Fig. 1. View of a specimen, showing on the left side parts of two pseudo-ambulacral 

 areas. [ The number of pieces in the pseudo-ambulacral areas is about half 

 as great as represented in the figure.] 



Geological formation and localities. In the Keokuk limestone : Keokuk, 

 Iowa ; Nauvoo, Illinois. 



Pentremites bipyramidalis (n.s.). 



PLATE XV. FIG. 2. 



GENERAL form bipyramidal, the greatest diameter being a 

 little below the middle at the base of the pseudo-ambulacral 

 areas, distinctly pentagonal : base truncate, narrow, sub- 

 angularly concave on each side. Basal plates short, little 

 more than one-fifth the entire length of the body, slightly 

 concave on their upper edges. Radial plates sublinear, gra- 

 dually widening from the base to the base of the pseudo- 

 ambulacral areas, and becoming very gradually narrower 

 above, prominently angular along the middle of the lower 

 part : branches slender ; the two adjacent ones, with the 

 interradial plate, presenting an elongate lanceolate form. 

 Interradial plates short-lanceolate, reaching to the summit, 

 which is truncate. Pseudo-ambulacral areas narrow, almost 

 linear, somewhat deeply seated particularly above the mid- 

 dle ; each marked by a double row of about forty-eight 

 squamiferous pore-pieces. Summit narrow ; mouth central, 

 somewhat semicircular ; ovarian aperture obscure ; anal 

 aperture large, elongate. Surface doubly striated. 



Fig. 2. View of the antero-lateral side. [ The summit and base of the figure are too 

 wide, and the number of pseudambulacral pieces represented is little more 

 than one -half the true number.] 



Geological formation and locality, The horizon of the Keokuk limestone, 

 State of Missouri. 



