44 The Auicrican Geologist. •^"'^'' ^^*^-- 
except from accident of fossilization- The original figure by 
James Hall in Geol. Wis., vol. i, (1861) p. 38, fig. 6, is evi- 
dently drawn from an average specimen of M. plana and is 
well enough represented to make identification of the common 
specimens not difficult. It shows no hinge teeth. The de- 
scription and figure by E. O. Ulrich in 19th Ann. Rep. Geol. 
Sur. Minn., p. 224, is not a fair one and represents an extreme 
of the alate posterocardinal region. The species as seen in an 
ordinary collection has not so little variation as he represents 
in its characters, and in fact his ]M. plana H. is only one ex- 
treme of wdiich M. similis Ulr. proposed at the same time (op. 
cit., p. 22.S) is the other. The ditTerences ascribed to these 
species are certainly not real and no others have been discov- 
ered. Modiolopsis similis Ulr. is again described in Final 
Rep. Geol. Sur. Alinn., vol. 3., p. 504. pi. 34, fig. i and 2 ; pi. 
42, fig. 19, with the suggestion that it "belongs to the line 
which finally produced M. modiolaris." At the same time a 
new genus, Eurymya Ulrich, is founded on 2^1. plana Hall. 
the chief characteristic of which seems to be "an obscene cardi- 
nal fold or tooth in the left valve and a corresponding depres- 
sion in the right," op. cit., p. 512. This character might in 
fact be seen on any one of the appressed black imprints which 
are always imperfect, but better specimens show more. Eury- 
mya is based on a fossil remnant of true characters of the 
species. Ulrich's two reconstructed figures {2y and 28, pi. 36, 
op. cit.) of AI. plana H. are not alike, being in dift'erent de 
grees imperfect. 
The evident fact that a not uncommon, apparently well pre- 
served fossil should be so subth- unlike the shell from which 
it originated, seems worthy of emphasis. It argues that the 
50 or more Ordovician species of the genus Modiolopsis as 
given by S. A. Miller's catalogue, and some other species of 
similar genera, with few probable exceptions, may have nor 
mally had hinge structures similar to that of M. plana H. and 
3'et appear as they do, to have been thin edentulous shells ; and 
that even a large collection of a species can not be relied upon 
to show true structures until the hinge teeth are either knowii 
or their absence on the shell has been carefully determined. 
]\Iany existing descriptions and figures are not to be accept- 
ed without reserve. 
