OSTRAOODA. 661 



EurychlUna subradiata.] 



inner side of the valve, we see nothing of the outer wall of the marginal area. The 

 last I regard as the most important difference, since, if the facts are correctly repre- 

 sented in Prof. Jones' figures it would remove his species from the typical section of 

 Eurychilina to that distinct group of species which is defined on a preceding page 

 in the remarks following the generic description. 



Variety indue v a, n. var. Plate xliv, Fig. 2. 



This subordinate name is proposed for a variety of this species that is rarely 

 associated with more typical specimens in the upper third of the Trenton shales, the 

 highest horizon in which this species is known to occur. The variety is a little 

 smaller than full grown specimens of the typical form, and more rounded in the 

 posterior outline. More striking differences however are seen in the marginal 

 area. This, instead of being concave and curved outward, is convex and incurved, 

 its width is less and more equal, the radii very indistinct and the terminal border 

 more sharply defined. These differences produce a form closely resembling the 

 Kentucky species jE/. granosa. We except of course the ornamentation, the two being 

 very different in this respect. 



Formation and locality — Ranges from the lower Trenton limestone to the upper third of the Tren- 

 ton shales; Minneapolis, St. Paul, Cannon Falls, and near Fountain, Minnesota. The species is not 

 abundant anywhere, only about twenty specimens In all having been seen. Variety incurva occurs as far 

 as If nown only at St. Paul. 



Eurychilina subeadiata Ulrich. 



PLATE XLIV, FIGS. 3, i. ia. 



Eurychilina subradiata Ulhich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiil, p. 126. 



Size. — Without marginal area, length 1.75 mm.; bight 0.9 mm.; thickness 0.50 mm. 

 With marginal area, length 2.32 mm.; bight 1.3 mm. 



Body of valves almost exactly semicircular in outline, with the surface highest 

 along an obtuse ridge-like prominence, running lengthwise across the central portion 

 of the valve and from the summit of which the surface descends with a distinctly 

 concave slope to the thickened dorsal edge; on the opposite or ventral side the slope 

 is more gently concave or flat; anterior extremity compressed; sulcus deep and 

 unusually wide, beginning a little within the dorsal margin and extending half way 

 across the body, its lower and posterior margins thickened and sharply defined; just 

 back of the sulcus a large round tubercle; surface appearing smooth in some speci- 

 mens, but usually it is pitted as shown in fig. 4. Marginal area nearly flat, the inner 

 edge rising abruptly and forming a low, sloping wall around the body, the outer 

 edge formed by a sharply elevated narrow border; posterior and ventral portions of 

 area holding about the same width, but at the anterior end it is usually much less; 

 external surface of area with more or less obscure radial furrows. Inner side of 



