287 



Description. " Shell obliquely oblong. The cardinal margin posterior 

 to the beak nearly straight, rounding gradually into the oblique posterior 

 margin. Anterior to the beak, the cardinal margin is deflected down- 

 ward, and then rounds into the strongly curved anterior margin of the 

 shell. Basal margin straight along that part of the shell which lies 

 directly opposite the straight cardinal margin; rising gradually toward 

 the curved anterior margin, and more rapidly towards the posterior 

 margin, which is most curved at the posterior extremity of the umbonal 

 ridge. Umbonal ridge most strongly defined on the cardinal side and 

 within about 10 or 15 mm. from the beak, almost disappearing into the 

 general convexity of the shell posteriorly. Mesial sinus practically 

 obsolete, although occasional specimens show a very faint indication of 

 the same accompanied by a scarcely perceptible concavity of the basal 

 outline. General convexity of the shell small. Concentric striations best 

 defined anteriorly, along that part of the shell which is anterior to the 

 oblique umbonal ridge. Anterior adductor depressions large and dis- 

 tinctly defined, although usually very shallow, owing to the thinness of 

 the shell. The interior of one of the valves is faintly striated posteriorly, 

 below the umbonal ridge, in a direction parallel to a line drawn from the 

 posterior termination of the umbonal ridge to a point half way between 

 the beak and the upper anterior margin of the shell." Foerste, 1914. 



Numerous references have been made to this species in the literature 

 and it has undoubtedly been misidentified many times. The only serious 

 study of this pelecypod is that in 1914 by Foerste whose description is 

 quoted above. Modiolopsis modiolaris as restricted by Foerste is a guide 

 fossil of the Pulaski shale in New York and in the corresponding hori- 

 zons southward in the Appalachians and west to the Ohio Valley. 



Occurrence. MARTINSBDRG SHALE (Fairview division). Tuscarora 

 Mountain, one and one-half miles southeast of McConnellsburg, Penn- 

 sylvania. 



Collections. Maryland Geological Survey, U. S. National Museum. 

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