LAMELLIBRANCniATA. 503 



Modiolopsts.] 



Type: M. modiolaris Conrad, sp. 



As here restricted and defined, this genus constitutes a well-marked group of 

 lower paleozoic shells. The oldest specieti!, 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 M. modiolaris 

 type. At the same time there existed elongate forms like M. arguta, having so much 

 in common with Orfhodesma that we cannot doubt that they indicate the primitive 

 stock from which Modiolopsis and Orthodesma were evolved. The M. arguta line 

 continued and fornied a reasonably complete chain through M. 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 M. 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. suhrecta 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 iniportant 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.f 



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 l'ala3ontology, p. 458; 1889. 

 + Kentucky Fossil Shells, p. 216; 1889. 



