52 BULLETIN 115, UNITED STATES NATIONAL MUSEUM. 



interbrachial plates marked by rows or clusters of pustules or wrinkles 

 more or less radiately arranged, or by radiating striae crossing the 

 sutures to adjoining plates and tending to form geometrical figures, 

 which when the striae are fine may become intricate and form several 

 included triangles; more or less prominent central nodes may be 

 formed by coalescence of these radiate structures, or nodes may be 

 wanting. Tegmen more or less lobed, ventricose or low convex, 

 with plates smooth, granular, rugose, bearing small spinous tuber- 

 cles, or rarely strong spines. Pinnule openings few and inconspicu- 

 ous, often obscured by the rugose sculpture. Arms 15 to 20, excep- 

 tionally more or fewer. 



The above description applies to the leading form of this genus 

 in the Louisville area, for which no less than 22 species and 2 varie- 

 ties have been named, 15 of the species by Miller and Gurley, and 7 

 species with 2 varieties by other authors. The wide bursiform 

 calyx, constricted above and truncate below, with strong pustulose 

 or striate ornamentation, imparts a -f acies which would well charac- 

 terize a strong and variable species. It is represented by numerous 

 individuals, among which may be found more or less difference in 

 superficial characters, producing just such an assemblage of minor 

 variations as is to be expected in a dominant species, flourishing 

 abundantly under favorable conditions, at the acme of the group 

 to which it belongs and on the eve of its extinction. 



The instability of characters in this form is evidenced by the 

 frequent occurrence of unsymmetrical conditions among the plates 

 of the calyx: those of the radial series are often larger or smaller in 

 one or two rays than in the others, as seen in Miller and Gurley's 

 figure of D. greenei-^^ or the interrays may be unequal, the posterior 

 one frequently the largest; and all the principal plates — basals, 

 radials, and interbrachials — are subject to considerable irregularities 

 in size as between specimens otherwise identical. This unequal 

 growth of plates which are usually pentamerously symmetrical in 

 the crinoids produces a certain asymmetry in the contour rather 

 frequently observed among the specimens of this form, by which 

 the calyx will be higher or more ventricose in one part than another. 

 Suppression of an entire ray occasionally occurs, as shown by Rowley's 

 figure in Greene (pi. 47, fig^ 2), and by specimens in my collection; 

 also of one primibrach, as in Greene (pi. 57, figs. 16, 17, 18). 



The numerous species which have been described under this form 

 depend for the most part upon minor differences which are to be 

 found in any vigorous and prolific species, notably those in the 

 number and grouping of arms, upon which most of Miller and Gurley's 



>« Bull. 4, pi. 31, fig. 10; also herein, pi. 14, fig. 7. 



