HEXACRINID^. 769 



Dichocrinus crassitestus White. 

 Plate LXXVI. Figs. 6a, I, c. 



1863. White; Proceed. Bost. Soc. Nat. Hist., p. 19. 

 1881. W. and Sp. ; Revisiou, PaliEocr., Part II., p. 83. 



Of moderate size. Calyx up to the arm bases once and a half as high 

 as wide; cylindrical along the radials, obconical below; the posterior side 

 extended into a short conical tube, supported by the large anal plate ; 

 ventral disk depressed-convex; plates perfectly smooth; the suture lines 

 rather obscure. 



Basal cup fully one third the length of the calyx; conical, the sides 

 straight, the lower end sufficiently truncated to form the columnar attach- 

 ment. Eadials twice as long as wide, the sides parallel or nearly so, the 

 lower margins convex (at the anterior plate obtusely angular) ; the upper 

 faces excavated to three fourths their width, slightly sloping at the ends. 

 Costals two, linear, their combined length less than their width, and so 

 closely anchylosed that the lines of union are seen with difficulty ; upper 

 face of the second plate very obtusely angular. Distichals 2 X 10, nearly 

 one half narrower than the costals, equally short, and anchylosed in a similar 

 manner. Arms four to the ray, rather delicate, twice as long as the calyx ; 

 composed throughout of a single series of short joints, united by parallel 

 sutures. Pinnules of moderate size. Anal plate of the same width as the 

 radials, but considerably longer ; its lower face distinctly angular, the upper 

 truncated, slightly sloping to the sides, and supported by three medium-sized 

 plates, which form the base of an anal tube. The tube is rather short, 

 conical, placed at the margin of the disk, but bending inward to a position 

 between the arms. The plates of the interradial series are not numerous ; 

 the three in contact with the radials are followed by two interambulacrals, 

 and these by a third. Column small, composed of circular joints, which 

 increase in length downward, the nodal joints longest and widest. 



Horizon and Locality. — Upper Burlington limestone ; Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



Remarks. — This species differs from all others heretofore described in 

 the form of the costals and distichals, and in having an anal tube. 



97 



