PALEONTOLOGY OF IOWA. 629 



" BASAL pieces five : four of them precisely of the same 

 form and size ; the fifth, on the anal side, a little larger and 

 different in form, alternating with the first radial pieces. 

 Radial pieces four to each ray, including the axillary piece. 

 All these, except the last, alike, and united to the interradial 

 pieces, which number twelve to thirteen on each regular 

 side. On the anal side, the upper extremity of the basal piece 

 is united to two small anal pieces, and consequently has two 

 articulating faces more than the four other basal pieces, none 

 of which join directly to the interradials. The fourth radial 

 or auxiliary piece gives origin to two rays, each of which is 

 composed of four articulations. Between these rays, which 

 bifurcate once or twice more, are ordinarily three interaxil- 

 lary pieces. The presence of these pieces seems to authorise 

 us to believe that the calyx of the species of this genus is 

 extended beyond the origin of the arms, the last ramifica- 

 tions of which may be as many as sixty. Arms composed of 

 very thick articulations, having a little median appendage 

 at the lower side, which corresponds to a small notch on the 

 upper side of the plates upon which they rest. Last articula- 

 tions of the column very fine, thin and regular." 



There are in the collection before me some half dozen species, which, though 

 apparently referable to this genus of DE KONINCK and LE HON, present in their 

 fundamental structure certain differences from the generic formula given by these 

 authors. They exhibit, moreover, among themselves some diversity in the de- 

 velopment of the basal portions ; and we may suppose that the structure of this 

 part has been misapprehended. 



In the diagram cited above are represented five basal plates, one of which is larger 

 and different from the others in form. In our specimens these plates are sometimes 

 conspicuously separated from the column by a narrow rim which is formed by a 

 lower series of plates, the five angles of which become distinctly visible between 

 the bases of these five plates, which thus become properly subradials. So closely 

 united are the plates of this lowest series, that in the perfect specimens it has been 

 difficult to trace the subdivisions. In some fragments of columns, however, preserving 

 the first plates, and one of the second series, the lower part is very clearly seen to 

 consist of three plates closely united, forming a narrow rim on the exterior, which is 

 elevated in five angles at the junction of the second series. These basal plates are 

 much thicker and very prominent towards the middle, with an abrupt depression in 

 the centre, forming the base of the visceral cavity. These plates therefore constitute 



