40 The American Geologist. •^"'^'' ^^°-- 
same shell has vanished. Both imperfection of fauna and de- 
ceptive fossil structure may accompany the specimens as they 
are collected. 
A special case of this kind, and one worthy of note as an 
example of a subtle deceptiveness which may exist in a large 
number of fossils of this kind in the Paleozoic rocks, is pre- 
sented by the pelecypod, ModiolojDsis plana Hall, a species 
from the Galena (Trenton) stage of the Ordovician. It is 
yery deceptive in features arising on account of the shells 
having dissolved while the sediment was solidifying. The 
species is further im])ortant as the basis for the genus Eur}- 
mya, into the definition of which the deceptiveness of the fos- 
sil has entered. The fossil appears one of the simplest, while 
in reality it is one of the most complex owing to the manner 
of fossilization. It appears so well preserved that the biologist 
would, quite according to custom, write descriptions of the 
forms to serve as definitions of the species, vet, in fact, such 
definitions are in this case not correct and a more correct 
description is here given of the original shell characters after 
careful reconstruction. 
Modiolopsis plana Hall is not a rare fossil and occurs 
widely in the lower beds of the Galena stage in Wisconsin, Illi- 
nois, Iowa and Minnesota, and should be seen in any good 
coUection of fossils of this region. The same manner of pre- 
servation prevails in distant localities and has only exception- 
ally been found to vary. As to the usual preservation, these 
fossils are black imprints in limestone and represent both 
external molds and internal casts of the valves. These vary 
from flat to strongly convex, or respectively concave surfaces 
upon which as a rule the shell characters are indistinct, as if 
from a thin shell ; l)ut when distinct the muscle scars and 
varices of growth are superimposed. In a split piece of lime- 
stone one may find for example the opposing imprints of a 
single valve of which the concave mold bears both growth 
varices and muscle scars and the convex internal cast bears 
also both muscle scars and growth varices. Placing the two 
imprints together, no space of measureable width remains, as 
if the shell had been a mere film in thickness, — a fact which 
may be overlooked by the collectors unless pains be taken 
to match the several ^asts and molds. The imprints were 
