494 PALAEONTOLOGY OF ILLINOIS. 



MOLLUSC A. 



LAMELMBRANCHIATA. 



MODIOLOPSIS STJBNASUTA, M. and W. 



PI. 23, Fig. 9 a, b. 

 Modiolopsis subnasuta, Meek and Wortiien, 1870. Proceed. Philad. Acad. Sci., p. 41. 



Shell rather small, elongate, narrow and slightly arcuate, rather dis- 

 tinctly convex, the most gibbous part being along the posterior umbo- 

 nal slopes above the middle of the valves ; dorsal and ventral margins 

 slightly diverging posteriorly, so as to make the widest (highest) part 

 of the valves uearest the posterior end, while the most sinuous part of 

 the ventral margin is a little in advance of the middle; anterior end 

 narrow, a little produced, with an obiique forward slope of its upper 

 margin, to its narrowly rouuded edge; posterior margiu somewhat 

 cuneate, with an oblique truncation more or less convex in outline, to 

 the posterior basal extremity, which is narrowly rounded ; cardinal 

 margin long, and a little arched ; beaks much depressed, and placed 

 rather nearer the anterior edge than the middle; posterior umbonal 

 slopes prominent, and forming an obtuse ridge extending obliquely 

 backward to the posterior basal edge of each valve ; anterior muscular 

 scar comparatively large, round, shallow, and placed near the edge of 

 the valves ; small pedal muscular scars distinct above those of the 

 anterior adductors; surface of internal cast, showiug moderately dis- 

 tinct, irregular, concentric undulations, which are most strongly defined 

 below, aud in front of the posterior umbonal slopes, on the flattened or 

 concave flanks. 



Length, 1.31 inch ; hight, 0.50 inch ; convexity, 0.40 inch. 



This is a neat symmetrical shell, resembling M. nasuta, Conrad, (sp.) 

 but differs in having the narrowed anterior less j^roduced, the beaks 

 being placed farther forward ; while its posterior end is broader, aud 

 obliquely truncated, instead of being rounded. Its general outline is 

 more like that of Orthonota contractu, Hall, (Palseont. 1ST. Y., Vol. I, PI. 

 32, tig. 8,) though its lower margin is less distinctly sinuous, its beaks 

 more depressed, and its posterior margin oblique; while it wants the 

 oblique dorsal wrinkles seen on casts of that shell, being a true Modio- 

 lopsi.s. 



On comparison with foreign species, our shell will be found to be very 

 closely allied to the Upper Silurian species, M. platyphyllus, of Salter. 



