258 The American Geologist. May, i89o 
Hall the third radial is a larger and stronger plate than is 
usually observed in the other, and the general form of this 
species is more robust. We have here, apparently, the cul- 
mination of this generic form which becomes extinct with the 
Keokuk period. One unusually large individual before us pre- 
sents a remarkable development of the third primary radial. 
Four of these are extremely protuberant, the nodes projecting 
three-eighths of an inch from the surface of the calyx. The 
specimen measures an inch in hight by two and one-fourth 
inches in breadth. A tendency toward the same structure, (ap- 
parently due to age) has been noted in other specimens. 
In Agaricocrinus americanus Roemer we have a form pre- 
senting many variations of structure. Some of these have 
been mistaken for specific differences giving rise to the syno- 
nyms A. bullatus Hall. A. excavatus Hall. A. nodosus M. & W. 
A. tuberoaus Troost. Among the specimens before us, usually 
referred to this species, are two forms which, as we believe, pre- 
sent more than varietal differences. While the distinctions 
giving rise to the above synonyms were largely superficial, in 
the present case the differences in structure seem to have a 
deeper significance. We shall attempt but a brief notice of 
them here. 
In the typical form the basal concavit}^ is said to involve 
the entire radial series of plates, while Messrs. Wachsmuth 
and Springer state :* '^Thc differences in the form of the calyx 
are modifications in geological succession. Species with convex 
sides are confined to the Waverly group and to the Burlington 
limestone: species from the Upper Burlington are truncate below, 
or slightly convex, rarely concave, while the Keokuk species ivith- 
out exception are deeply concave in thebasal region.^' 
We have a number of specimens in which the basal concav- 
ity is very shallow and does not involve the third radials and 
frequently only the basal plates. The third primary radials 
are more or less prominently convex, heavy plates as are also 
the interradials. In the postero-lateral rays one of the second 
radials is oblique and unequally quadrangular while the other 
(in the right ray) has its upper acute angle truncated by one 
of the interradial plates. In this particular it agress with A. 
whitfieldi Hall, from which it differs, however, in being much 
less concave below and more protuberant in the anal region. 
■^Revision Palseocrinoidea, Part, ii., p. 109. 
