LAMELLIBRANCHIATA. 513 



Modlolopsls nana.l 



as the shell; anterior end very small, sharply rounded above, curving backward into 

 the slightly convex, medially almost straight, basal margin; posteripr edge gently 

 curved, truncate, nearly vertical; strongly convex below; above turning abruptly 

 into the hinge line. Beaks small, but little incurved, not prominent, about one- 

 sixth of the length of the shell behind the anterior extremity. Umbonal ridge mod- 

 erate, cardinal slope flat or slightly concave, ventral and anterior slopes depressed 

 convex. Surface marked with distant strong concentric lines of growth, and between 

 these with a finer set. In casts of the interior the anterior muscular impression is 

 well marked, not very large, vertical, situated in front of the beaks and close to the 

 cardinal margin. A little more than one-third of the scar is divided off above by a 

 distinctly impressed transverse line. Posterior scar indistinct, much larger than the 

 anterior, situated behind the center of the posterior cardinal slope. Pallial line simple, 

 rather distinct, not following the outlineiof the shell, being farther removed from the 

 margin in the postero-basal region than elsewhere. Hinge as shown in figure 28 on 

 plate xxxvi. Free casts of the interior of both valves, being without the hinge 

 plate, are longer in proportion to the hight than are the impressions of single valves. 



The shape of the shell will distinguish this species at once from all Minnesota 

 Lamellibranchiata except Matheria rugosa and Gyrtodonta affinis, both of which occupy 

 a higher horizon and have a different shell structure. 



I have before me ten more or less complete silipified shells from the middle Tren- 

 ton or " Orthis beds " of Tennessee and Kentucky. These belong to a species that is 

 closely allied to E. plana and which may be called Eurymya subplana, n. sp. The new " 

 species does not attain the size of the Minnesota form, and is not so high and more 

 oblique posteriorly, while the margin is less narrowly rounded in the postero-bagal. 

 region. The Cincinnati specie's, E. alata (Modiolopsis alata Ulrich) is a slightly 

 shorter and more compressed shell, with a more convex basal margin and different 

 anterior muscular scar. 



Formation and locality. — Lower limestone of the Trenton formation in Minnesota at Minneapolis, 

 St. Paul and Cannon Tails.. In Wisconsin the species seems to be restricted ta the "Lower Blue" 

 limestone at Janesville, Beloit and Mineral Point. 



Mm. Rug. Nos. 749, 757, 5011, 5012, 5013, 5062, 5358, 5669, 5834, 8312. 



Genus ACTINOMYA, n. gen. 



Modiolopsis (part.), of various authors. 



Shell ovate, more or less elongate, narrowing anteriorly. Valves moderately 

 ventricose, fitting each other tightly. Anterior end short, but not excessively so. 

 Base gently convex, occasionally straight, never sinuate. Mesial sulcus wanting. 



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