260 The American Geologist. May, i89o 
lateral ray. The tendency is clearly shown to be toward six- 
teen arms. 
The basal concavity, which is exceptionally deep, involves 
the whole series of radials up to and partially including the 
secondary radial so that when placed base downward the fos- 
sil rests upon the somewhat protuberant secondary radials. In 
this feature it somewhat resembles J., whiffieldi Hall. Above the 
secondary radials the plates are much smaller than in the ordin- 
ary form. These examples all agree in the somewhat depressed 
structure of the dome and very protuberant anal region. The 
proximal and radial dome plates are quite prominently tumid 
with the exception of the third radial which is generally 
smaller than the others and somewhat depressed. The second 
radials are the largest and most tumid plates of the dome in 
rays having two arms, while the succeeding plates are obscure 
or absent ; and in rays with more than two arms,two additional 
well developed plates occur. In rays having three arms, one 
of these plates is large and conspicuous while the other is 
small and crowded to one side ; and in four-armed rays the 
two plates are equally prominent and placed side by side. The 
general form of the body is depressed wheel-shaped, and owing 
to the deeply excavate base the capacity of the internal cavity 
must have been very small. 
In an examination as to the mode of origin of extra arms it 
was observed that, in those rays having normally three arms, 
the third arm arises from a single tertiary radial which trun- 
cates, usually, the upper and inner angles of the first second- 
ary radials. In four-armed rays this plate is divided vertical- 
ly thus forming two tertiary radials from each of which 
springs an arm. Sometimes the arms have the appearance of 
springing from the sides owing to being pressed outward by 
those within. 
Nearly if not quite all the examples with sixteen arms from 
this locality are derived from the same geological horizon, 
viz. : the "Lower Crinoid Bed, (No. 2)" near the base of the 
lower division, and associated with the subexcavate form above 
noticed. 
KeoJcuk, Iowa, March 8, 1890. 
EXPLANATION OF PLATE 
Fig. 1. — Agaricocrinus wortheni Hall. 
Basal view of specimen showing deep concavity and hexagonal form 
of second radials. 
