502 THE PALEONTOLOGY OF MINNESOTA. 
{Modiolopsis. 
the configuration of these parts in the latter is much more like what we see in the 
Crytodontidw. Another feature in which the Modiolopside resemble the Cyrtodontide, 
and one that, so far as | am aware, has never been noticed in Modiola nor Mytilus, 
is the presence on the inner surface, at any rate‘of all the thick shells, of one or two 
obtuse ridges extending from the beaks obliquely backward and toward the ventral 
margin, producing corresponding more or less well-marked furrows on casts of the 
interior. 
Finally, there is to be urged that it is only a few shells, like Modiolopsis modio- 
laris and M. concentrica, in which the anterior end is narrow and unusually short, 
and a byssal sinus present, that exhibit any striking resemblances to either Modiola 
or Myoconcha. No one would, I believe, say this of elongate shells like M. arguta and 
M. angustata, and when it comes to Orthodesma, which can be shown to have origin- 
ated in the same stock that produced Modiolopsis, all agree in removing that genus 
far from the Mytilide. : 
The many points of agreement that may be noticed between the Modiolopside 
and the Cyrtodontide probably indicate a close union of the two groups in times 
preceding the Chazy; but, as far back as our knowledge now extends, there prevailed 
at least one important distinguishing feature. Namely, there existed a difference in 
the shell structure which, though its exact nature is unknown, is nevertheless clearly 
evidenced by the appearance of the two groups of fossils when they are preserved in 
soft shales, the shells of the former always being covered by a black or dark film 
never seen on the latter. 
Genus MODIOLOPSIS, Hall. 
Modiolopsis (part.), HALL, 1847. Pal. New York, vol. i, p. 157. 
Shell more or less elongate, usually subovate, widest posteriorly; valves moder- 
ately ventricose, closing tightly all around. Beaks small, near the anterior extremity; 
umbones depressed by a flattening or depression which crosses the valves obliquely 
and widening causes a straightening or sinuation of the basal outline. Hinge of 
moderate strength, rarely straight, generally somewhat arcuate, without well-marked 
teeth; an obscure oblique thickening beneath the beak of one valve and a corres- 
ponding depression in the other occasionally distinguishable. Ligaments linear, 
external and internal, chiefly the former. Anterior adductor impression subovate, 
large, deep, sharply defined on the inner side, occupying the greater part of the small 
anterior end. Posterior scar very faintly impressed, large, subcircular, situated near 
the center of the posterior third of the cardinal slope. Pallial line simple. Anterior 
pedal muscle forming a minute pit in the under side of the hinge plate beneath the 
beak. Posterior pedal muscles large, attached just above and in front of the adductor, 
