~ 
r4 ~ ‘THE AMERICAN MIDLAND NATURALIST 
Through these studies and collections it has been clearly shown 
that the species of this genus represent a most wonderful plasticity, 
and demonstrates more clearly than ever the great difficulty of 
determining what really does in nature constitute a good “‘species.”’ 
It is shown that with every radical (and sometimes relatively slight) 
change in the deposition and environment in which the various 
forms we call species, lived, there followed an equally great modi- 
fication of the forms, and that under these conditions they were 
remarkably uniform and persistent in their form and character. 
There is no other group of ancient life perhaps which shows equally 
well the great plasticity of species (aside from the Stromatoporas 
and their allied forms) than do the forms of the genus Atrypa. 
Specific names are here given toecertain forms of the genus more 
for the sake of convenience in reference and to facilitate the further 
studies of others, rather than to the belief that they always con- 
stitute true natural species—in fact I am convinced that in nature, 
relatively few natural ‘“‘species,’’ such as have been generally 
regarded as such, really exist. 
_ Nearly all the Atrypae described in this paper save A. owenensts, 
and A. subhannibalensis, have been referred by geologists to 
‘Atrypa reticularts. % 
* In the following description of forms it will often prove difficult 
for the student to fully appreciate the importance of the differences 
existing between them without good illustrations being shown; 
this, however, will be overcome in the final Monograph of which 
this paper is to form a part. 
DESCRIPTION OF SPECIES. 
Atrypa owenensis N. Sp. 4 
Plate Vill. Pigs 12—14% 
Shell small usually not over half inch in width; length equal to 
or considerably greater than width. 
Valves usually greatly convex and often nearly equally so; 
beak sharply incurved; plications rather coarse and transverse 
annulations or lines of growth prominent. This species is generally 
unusually constant in form and size, and has a certain phase sug- 
gesting A. zmpressa of Hall. This form is gregarious in its habits, 
and appears to be restricted in its range to a certain limestone bed 
of the Upper Hackberry Group (the ‘‘Owen substage”’ of Fenton), 
Owens Grove (south exposure), Cerro Gordo County, Iowa. 
