Carboniferous Rocks of the Mississipjn Valley. 279 



or denticulate rim. The ridges upon the plates are fewer 

 than in the typical forms;, the principal ones extend in 

 rigid, elevated lines entirely across the plate, and the oth- 

 ers are interrupted, and form rounded, bead-like elevations 

 in the intervals between the centre and sides of the plates. 

 The plates are less convex in the centre, and not deeply 

 indented at the sides. The dome maintains the form of 

 that in the described species. 



The features here noticed, if observed without consider- 

 ing intermediate varieties, might lead to the inference that 

 it is a distinct species. 



In the collection before me, there are numerous individ- 

 uals of the typical form and surface markings, while oth- 

 ers have these markings so far subdued as to present only 

 the denticulate margin, and another form shows the plates 

 strongly elevated in the centre, with a few sharp, angular 

 ridges, for the most part one to each face of the plate, and 

 the intermediate spaces along the margin deeply indented 

 or excavated, leaving the central prominence with a much 

 smaller area than the prevailing forms of the species. 



Geological formation and locality. Burlington lime- 

 stone, Burlington, Iowa. Collection of Dr. O. Thieme. 



Actinocrinus ventricosus var. reticulatKs (n. s.). Form of 

 body as in the original of the species, or even more spread- 

 insr; structure above the first radials undetermined. In- 

 terradial plates in ranges of one, two, three, and two. 

 Surface of plates moderately convex, and marked by low, 

 sharp ridges, which at their joining or crossing leave hexag- 

 onal or polygonal depressed areas between. These ridges 

 are of equal strength on all parts of the surface of each 

 plate and on all the plates. These impressed spaces give 

 a reticulate aspect to the surface of the crinoid. 



It is possible that this variety, of which we yet know no 

 perfect specimens, may prove a distinct species. 



The Actinocrinus subventricosus of McChesney is of a 



