New Lamellibra7ichiata. — Ulrich. 275 
end views of same, d, cast of the interior of the average size, siaowing 
the anterior muscle scar and deep impression just behind it. 
Shell small, tranversely elongate subovate, the length a lit- 
tle more than twice the hight. Valves strongly convex, with 
point of greatest convexity near the middle, the surface slop- 
ing rapidly from there to the anterior and posterior extremi- 
ties. Cardinal and basal margins subparallel, diverging very 
slightly posteriorly, the former very gently arcuate, the 
latter broadly sinuate. Anterior end very short, con- 
tracted beneath the beaks, and narrowly rounded. Pos- 
terior margin oblique, the basal half the most prominent 
and strongly curved. Beaks small, nearly terminal, um- 
bonal ridge obtuse, but generally well developed, defined on 
the upper side by the flat or slightly concave cardinal 
slope, and on the other by the depression of the valves, causing 
the sinuate basal margin which is such a characteristic feature 
of the typical species of the genus. The sinus and depression 
are comparatively stronger in this specie than usual. Surface 
of shell with very fine concentric strice and a limited number 
of stronger sub-lamellose lines of growth. Anterior muscular 
impression not deep, but bordered on the inner side by a strong 
clavicular ridge situated just in front of and below the beaks. 
Length of a large specimen, 27 mm. ; greatest hight (in the 
posterior half) 13 mm. ; hight at beaks, 10 mm. ; greatest con- 
vexity of both valves, 11 mm. In a small specimen these 
measurements are respectively, 14, 6.3, 5, and 4.5 mm. 
This species is related to M. modiolaris Conrad, but is readily 
distinguished by its smaller size, comparativelj'' greater con- 
vexity, stronger umbonal ridge, and more pronounced mesial 
depression beneath it. The two species differ also somewhat 
in their outlines. 
The specific name is given in honor of Mr. S. A. Miller, who 
is one of the few palaeontologists that have taken pains to col- 
lect and study the Cincinnati lamellibranchs. 
Position and locality : Cincinnati group, on the hills about Cincin- 
nati, Ohio, from about 300 to 400 feet above low water in the river. 
