550 GEOLOGICAL SURVEY OF IOWA. 



The base, instead of presenting the rotund outline with central depression, is turbi- 

 nate or basin-form, gradually enlarging to the base of the arms. The radial and second 

 radial plates are articulated in the same manner as in ZEACRINUS; and a marked 

 feature, of at least half a dozen species, is, that the second radials, which are often 

 long, are concave or compressed at the sides, swelling out above and below, while 

 the lateral articulating margins are thin and spreading at the sides beyond the convex 

 portion. The arm-plates, though arranged in single series as in ZEACRINUS, are 

 nevertheless more nearly wedgeform, by having one side much longer than the other. 

 The anal plates range from one to six, or perhaps more; while no interradials have 

 been observed. 



From the differences of habit, and certain peculiarities, I have thought proper to 

 propose a distinct generic term to include these species. 



SCAPHIOCRINUS ( n. g.). 



FIG. 70. 



GENERIC FORMULA*. 

 Basal plates, 5. 

 Subradial plates, 5. 

 Radial plates, 2 X 5 = 10. 

 Anal plates, 1 to 6 or more. 

 Interradial plates, 0. 



Arms, 2 X 5 = 10, two from each ray, simple 

 or bifurcating. 



BASAL plates five, small : subradial plates five, small, 

 hexagonal except that on the anal side, which is truncated 

 above by the first anal plate. First radial plates pentagonal, 

 with the upper side straight or slightly concave : second radial 

 plates pentagonal with the lower side straight, often much 

 elongated and the plate contracted or concave on each side. 

 Anal plates four or more. Interradials none. 



Arms double from their origin, or, rarely, with the anterior 

 one simple for some distance above the upper radial, con- 

 sisting of a single series of plates which are longer on one 

 side than the other, and often wedgeform with tentacula 

 originating on the longer side. 



* The accompanying diagram is an illustration of the structure of S. simplex, which 

 I have selected as the type of the group. 



