34-4 PALAEONTOLOGY OF ILLINOIS. 



, Meek and Haydcn's Pahvant. Upper Missouri, as M. subquadrata, of Shumard. 

 In addition to its greatly smaller size, however, it differs in having its valves 

 more convex in the postero-dorsal region, and its hinge more arched. 

 Locality and position : Same as last. • 



Myalina recurvirostris, M. and W. 



PI. 26, figs. 9a, 95, 9c. 



Myalina recurvirostris, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 456. 



Shell of medium size, moderately thick, obliquely subtrigo- 

 nal, distinctly inequivalve (the left valve being more convex 

 and thicker than the other) ; very gibbous and narrowly 

 rounded along the umbonal slopes. Hinge nearly or quite 

 straight, about equaling in length the height of the shell in 

 young specimens, but proportionally shorter in adult exam- 

 ples. Posterior side compressed; its margin a little convex 

 in outline, ranging nearly at right angles to the hinge above, 

 and rounding gradually into the narrowly rounded base below; 

 anterior side a little concave in outline, and ranging at an 

 angle of about 55 ° with the hinge. Beaks small, teminal, 

 pointed, that of the left valve being twisted so as to make 

 nearly one entire turn at the extreme point, which is directed 

 obliquely forward; beak of right valve less prominent, nearly 

 straight, and directed forward. Surface of left valve marked 

 by distinct lines, and imbricating laminee of growth, which are 

 quite prominent on the anterior side and near the cardinal 

 margin behind; right valve more obscurely marked. Liga- 

 ment area comparatively narrow, and marked by a few distinct 

 longitudinal furrows. Anterior margin immediately under the 

 beaks thickened, so as to present a broader area than the hinge, 

 from which it is separated, in the left valve, by an oblique 

 groove, and in the other by a corresponding prominence. Pos- 

 terior muscular impression large, elongated or narrow-subovate, 



