50 
PALAEONTOLOGY OF NEW YORK. 
CAL LON EM A, n. gen. 
Shell subglobose, turbinate or ovoid-conical. Volutions rounded or sometimes 
subangular above and below; outer lip apparently thin; columellar lip 
thickened and spreading over the volution above and extended below; 
axis umbilicate. 
Surface marked by fine even striae which extend equally over the volutions, 
rarely divided, and sometimes merging into the ordinary striae of growth, 
and extending into the umbilicus. 
This generic name is proposed to include such forms as Isonema bellatula , 
Isonemo Lidias (if these two prove distinct), Pleurotomaria ( Isonema) imitator , and 
similar forms; the chief external characters being the sharply elevated, even, 
thread-like concentric striae traversing the volutions above and below, a distinct 
columellar lip, and an umbilicate axis. 
I had accepted, without critical examination, the determination of Mr. Meek 
(Meek & Worthen), referring Loxonema bellatula to the genus Isonema; but while 
this volume is passing through the press, a comparison of the generic descrip¬ 
tion and the characteristic figure of the type species {I. depressa ), shows that this 
reference cannot be sustained. The generic description of Isonema is as follows: 
“ Shell depressed subglobose, turbinate, or conical-subovate, obtusely angular 
around the middle of the body-whorl. Aperture subrhombic; outer lip thin, 
entire; inner lip a little flattened or impressed in the umbilical region, appar¬ 
ently for the support of an operculum, very thin, or scarcely continuous above; 
axis imperforate. Surface ornamented with transverse, very regular lines on 
the upper side of the volutions. 
In 1865 we proposed the name Isonema for this type, as a subgenus under 
Holopea. Farther comparisons, however, have since led us to regard it as 
generically distinct from Holopea, from which it differs in its angular instead 
of rounded volutions, as well as in its imperforate axis, flattened inner lip, 
rhombic instead of rounded aperture, and strong regular lines of growth. 
From Pleurotomaria it will be at once distinguished by its entire lip, and the 
absence of a revolving band on the whorl. From Cyclonema it differs, in being 
entirely without the characteristic revolving lines of, that genus, and marked 
by strong transverse striae. 
In addition to the typical species here described, this genus includes 
I. bellatula (Loxonema bellatula, Hall : Fifteenth Heport Regents Univ. N. Y., 
p. 163, figs. 4 and 5).”— Geol. Survey of Illinois, Yol. Ill, pp. 442, 443. 1868. 
