686 GEOLOGICAL SURVEY OF IOWA. 



torn of the subradial plates, to the lower extremity. The solid basal portion some- 

 times presents a central longitudinal space filled with different material, with ra- 

 diating and dichotomizing branches of the same substance, which appears to have been 

 originally vessels reaching nearly to the exterior margin of the plates. The base of 

 the visceral cavity often occupies a small space at the top of the basal plates. The 

 arms are in like manner arranged as in the genera cited; and it is a closely allied 

 form, which will probably be traced through other species to more intimate relations. 

 It is probable that the species of this type were fixed in their young state, and 

 became free as they grew older. 



Agassizocrinus gibbosus ( n. s.). 



PLATK XXV. FIG. 6 & 6 b. 



BODY ovoid, protuberant below and at the subradial plates, 

 somewhat contracted at the base of the rays. Base solid, 

 preserving suture-lines and a scarcely defined indication of 

 the column-attachment : subradial plates large, gibbous, 

 about once and a half as long as the base. First radial plates 

 small, pentagonal, with one of the upper angles slightly 

 truncate. Second radials pentagonal, a little longer than the 

 first radials. Arm-plates short, quadrangular. Anal plates 

 four ; the lower one large, subquadrangular with the upper 

 lefthand angle truncated; the three upper ones small. 



This species differs from the A. daciyliformis of TROOST in the generally smaller 

 form, smaller and protuberant base and gibbous subradials, while it is more distinct- 

 ly contracted at the base of the radials. 



Fig. 6. View of the anal side of the specimen. Fig. 6 b. Basal view of the same. 

 FIG. 114. 



The accompanying diagram illustrates the structure 

 species, the form and proportions of the plates, 

 etc. 



Geological position and locality. In the Kaskaskia limestone : Chester, 

 Illinois. 



