502 THE PALEONTOLOGY OF MINNESOTA. 



the configuration of these parts in the latter is much more like what we see in the 

 ( 'ri/todontidic. Another feature in which the Modiolopsid<(> resemble the Cyrtodontidce, 

 and one that, so far as 1 am aware, has never been noticed in Modiola nor Mi/tilus, 

 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 pixxjuced Modiolopsis, all agree in removing that genus 

 far from the Mytilidii'. 



The many points of agreement that may be noticed between the Modiolopsidce 

 and the Cyrtodontidce 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; 

 urnbones depressed by a flatbening 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. 



