480 GEOLOGICAL SURVEY OF IOWA. 



The successive ranges of plates in the species known, 

 present the number of three in the first, six in the second, 

 twelve in the third, etc. 



The spedes of this type have so much the structure of ACTINOCRINUS, that a 

 little extension of the generic characters of that genus would include them. Our spe- 

 cimens, however, approach more nearly the Genus PRADOCRINUS of DE VERNEUIL, 

 to which they are closely allied in structure; but having a scarcely visible calyx, 

 they do not correspond with the description and figures given. 



The Genus MEGISTOCRINUS, proposed by OWEN and SHUMARD in 1852, has essen 

 tially the structure of the species under consideration, in the basal, second and third 

 series of plates, as well as in the relations of the parts of the calyx. 



A farther comparison of specimens, however, shows that these forms are but 

 modifications of the typical generic form of ACTINOCRINUS, and do not appear to 

 me to be separable upon fundamental structure. The illustrations under the Genus 

 ACTINOCRINUS of the Burlington limestone, in this volume, will show more clearly 

 the relations here indicated. 



Actinocrinus (Subgenus Megistocrinus ) latus (n.s.). 



PLATE I. FIG. 1 a, b. 



BODY depressed spheroidal or broadly urnshaped : base 

 broad, nearly flat or slightly concave towards the column ; 

 base composed of three closely joined or anchylosed plates, 

 presenting a flat hexagonal disk which barely extends beyond 

 the column. First radial plates hexagonal, wider than high, 

 base and upper margins straight and parallel, the base a little 

 the longer ; second and third radials hexagonal (one penta- 

 gonal), base straight. Each third radial supports on its upper 

 sloping sides two superradials, which, in the anterior and 

 antero-lateral rays, are each succeeded by a second super- 

 radial in the same series, and in the postero-lateral rays each 

 first superradial supports a pair of diverging second super- 

 radials, and each of these, two others in the same direction, 

 giving four ranges of superradials in those rays : these are 

 succeeded by the brachial and arm-plates, giving, apparently, 

 but four arms to the postero-lateral rays, while each of the 

 other rays has the same number. First interradials hexagonal, 

 as large as the second radials, succeeded by two others, each 



