126 Czncinnatt Society of Natural Ffistory. 
more or less prominent node behind it. A very broad convex 
border extends around the valves from the antero-dorsal to the 
postero-dorsal angle. This border is often striated in a radial 
manner, and in most cases terminated by a marginal ‘frill,’ or by a 
plain narrow border, usually directed slightly outward. The main 
body of the border, however, curves inward to near the plane of 
contact between the two valves, thus forming a deeply concave 
outer area. Hingement simple. Surface reticulate, granulose or 
smooth.’’ | 
Type, M. RETICULATA, n. sp. Trenton shales, Minnesota. 
‘*This genus is related to Prmitia, Jones and Holl, and to 
frimitiopsis, Jones. ‘The possession of an internal concave mar- 
ginal area separates it from the first, while the greater extension of 
the hollow area distinguishes it from the second. In Prmitiopsis 
the marginal hollow is developed only at the anterior end.” 
EURYCHILINA SUBRADIATA, DN. sp. 
Flate 1X, L185: 10,105) C, and 2a ae 
Valves semicircular, or elongate sub-elliptical in outline. Dorsal 
margin straight, nearly as long as the valves, terminating abruptly at 
both ends. Upper two-thirds of anterior margin straight, forming 
nearly a right angle with the dorsal edge, then curving gently into 
the slightly convex ventral edge. Posterior end evenly rounded. 
Body of valves semicircular, moderately convex, the point of great- 
est convexity along an obtuse ridge-like prominence (most distinct 
in the anterior half), running lengthwise across the central portion 
of the valve and from the summit of which the surface descends 
with a gently concave slope to the dorsal edge on one side and the 
beginning of the broad ventral border on the other. Sulcus deep 
and wide, beginning a little within the dorsal margin and extend- 
ing half way across the body, its lower and posterior margins 
thickened and sharply defined. Just back of the sulcus a large 
round tubercle. Surface smooth. Marginal area wide, its width 
nearly equal all around, the widest portion in the postero-ventral 
region. In the antero-dorsal region it becomes separated from the 
body of the valve by the intervention of a narrow triangular 
depressed space. Inner edge of area abruptly elevated above the 
