300 PALEONTOLOGY OF ILLINOIS. 



LAMELLIBRANCHIATA. ' 

 Genus MYALINA,. deKoninck, 1844. 



(An. Foss. €arb. Belg., p. 125.) 



Mtalina angulata, M. and "W. 



PI. 23, fig. 7 a, lb. 



Myalina angulata, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Sci., Phila- 

 delphia, p. 455. 



Shell rather large, obliquely subtrigonal, nearly equivalve, 

 very convex and distinctly angular along the umbonal slopes, 

 cuneate in the posterior and postero-dorsal regions. Anterior 

 margins abruptly inflected from the imbonal angles, so as to 

 meet on a plane at right angles to that of the valves; 

 nearly straight, and sloping obliquely backwards at an 

 angle of about 57° below the horizon of the hinge, to the nar- 

 rowly rounded basal extremity. Hinge straight, nearly or 

 quite equaling the horizontal length of the shell, and termi- 

 nating in a rather acute angle behind. Posterior margin form- 

 ing a broad convex curve from the narrowly rounded base up- 

 wards nearly to the hinge, thence curving gracefully outwards 

 to its extremity. Beaks terminal, prominent, or extending 

 above the hinge, incurved, and acutely angular; strongly com- 

 pressed in front and behind. Surface of cast retaining rather 

 obscure marks of growth. Height, measuring at right angles 

 from the hinge, about 2.55 inches; length, parallel to the 

 hinge, 2.57 inches; length from the beaks, parallel to the um- 

 bonal slopes, to the base, about 3.20 inches ; convexity of the 

 two valves, near 1.30 inches. 



The extremely compressed, prominent beaks, and remarkably convex angu- 

 lar umbonal slopes of this shell, will at once distinguish it from all the other 

 known species of the genus. It is also peculiar in having the posterior ex- 

 tremity of the hinge rather acutely angular, judging from the curve of the 



