172 Review of Geinitz on the rocks and fossils of Nebraska. 
i, 
either side of the shell, although they have been frequently 
examined under a magnifier with the view of determining this 
very question, in numerous perfect examples, entirely free 
the matrix, and showing clearly all the volutions on both sides 
to the very center. 
Nor is Prof. Geinitz’s reference of his little Murchisonia sub- 
teniata, to that genus, or family even admissible. It belongs on 
the contrary, to the genus Orthonema M. & W. (III. Report, vol. 
ii, pl. 31, f. 14), a group of small shells resembling Murchisonia 
in form and revolving markings, but differing in being entirely 
without the slit or sinus in the lip characterizing that whole family 
of shells. Prof. Geinitz did not see a sinus in the lip of his 
specimen, nor the corresponding revolving band, but thought 4 
flattened space between two little revolving ridges might bea | 
band, though he acknowledges that this view would indicate 
proportionally wide flat band. In order to settle this point, 
however, an excellent specimen of the species subteeniata, was 
borrowed from Dr. White, the state geologist of Iowa, obtained 
from exactly the same horizon in the western part of that state, 
and on examining this by a cross light, under a good magnifier, 
its lines of growth were seen passing straight across the whorls, 
without making the slightest flexure such as we see in 
sonia and other genera provided with a sinus in the lip. Seve 
ral allied species of this genus are known in the Coal-measures | 
of Illinois. 
His reference of such shells as the so-called Modiola squamost 
and M. acuminata of Sowerby (as illustrated in Prof. King’s 
Perm. Foss.), to the genus Aucella, is also believed to be done 
on questionable grounds. The genus Awcella, it will be remem 
red, was founded by Count Keyserling, upon a group of Ju- 
rassi¢ shells, differing from those mentioned above, not only i® 
wanting their broad cardinal area, marked with linear parallel 
cartilage furrows, but also in possessing a little abruptly projece 
ing anterior ear, defined in the right valve by a deeply impressed = 
linear furrow extending from the beak to the margin, and termr | 
nating there in a sharply cut byssal emargination. Now on eX 
amining Prof. —— excellent figures of testiferous examples 
of the English shells mentioned above, it will be seen that they 
show no traces whatever, of this little anterior ear, though his 
