LAMELLIBRANCHIIATA. 503 
Modiolopsis.] 
Type: M. modiolaris Conrad, sp. 
As here restricted and defined, this genus constitutes a well-marked group of 
lower paleozoic shells. The oldest species, so far as known, occur in the Birdseye 
and Black River divisions of the Trenton formation. Some of these are of an oval 
type that, by gradual modifications of the base, evolved species of the MZ. modiolaris 
type. At the same time there existed elongate forms like M. arguta, having so much 
in common with Orthodesma that we cannot doubt that they indicate the primitive 
stock from which Modiolopsis and Orthodesma were evolved. The M. arguta line 
continued and formed a reasonably complete chain through WW. nana, M. mytiloides 
Hall, M. angustata, and one or two undescribed species of the middle beds of the 
Cincinnati group, into M. concentrica H. and W., a common species of the upper part 
of that series of rocks in Ohio and Indiana, and into MW. excellens from equivalent 
strata in Minnesota. In this case the form was shortened, the anterior end particu- 
larly. In the M. modiolaris line, however, the changes were different. Here we may 
begin with M. similis, an oval form with the posterior end broadly rounded and widest. 
This seems to have gone over into an upper Trenton species (M. subrecta Ulrich, Ms.) 
having a much narrower posterior end—indeed, the back and base are nearly parallel. 
We next follow the type by easy stages through varieties occurring in the Utica 
horizon to the normal form of M. modiolaris. Much indeed might be said upon these 
not only interesting but important questions of evolution, and nothing would please 
me more than to be allowed to demonstrate the positions here outlined. But time 
and space are lacking, and the few points made are offered chiefly in the hope that 
the suggestions may stimulate students to researches in similar lines. The field is 
inviting and the results to be obtained all important. 
The relations of the genus to the other genera of the family treated of in this 
chapter will be discussed in the remarks following their descriptions. 
No comparison of Modiolopsis and Modiomorpha, Hall, has, so far as I can learn, 
ever been published. This is strange, since the species of the two genera are strik- 
ingly similar. As a rule it seems they are regarded as differing widely, but in what 
respects we are not informed. Mr. S. A. Miller, for instance, places them into two 
distinct families, but fails to state his grounds for the separation.* A mistaken idea 
seems to prevail—where it originated I cannot say—that the hinge of Modiolopsis 
has lateral teeth, and this is given as the principal difference between the two genera 
by Nettleroth.+ 
Now, let us see what differences really exist between them. Taking Modiomorpha 
concentrica as representative of the Devonian genus, we find that, so far as external 
characters are concerned, it would pass very well for a species of Modiolopsis. Even 
*North American Geology and Valwontology, p. 458; 1889. 
+Kentucky Fossil Shells, p. 216; 1889. 
