LAMELLIBRANCHIATA. 513 
Modiolopsis nana.] 
as the shell; anterior end very small, sharply rounded above, curving backward into 
the slightly convex, medially almost straight, basal margin; posterior 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 
distinetly 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 outline of 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 Cyrtodonta affinis, both of which occupy 
a higher horizon and have a different shell structure. 
I have before me ten more or less complete silicified shells from the middle Tren- 
ton or “Orthis beds” of Tennessee and Kentucky. These belong to a species that is 
closely allied to 2. plana and which may be called Hurymya 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-basal 
region. The Cincinnati species, 4. alata (Modiolopsis alata Ulrich) is a slightly 
shorter and more compressed shell, with a more convex basal margin and different 
anterior muscular sear. 
Formation and locality—Lower limestone of the Trenton formation in Minnesota at Minneapolis, 
St. Paul and Cannon Falls. In Wisconsin the species seems to be restricted to the ‘‘ Lower Blue” 
limestone at Janesville, Beloit and Mineral Point. 
Mus. Reg. Nos. 749, 757, 5011, 5012, 5013, 5062, 5358, 5669, 5834, 8312. 
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Genus ACFENOAFYA, 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. 
—33 
