PELECYPODA—PRIONODESMACEA. 



453 



LXXV. Myalina De Koninck. 



Inequilateral, inequivalve, obliquely ovate, slightly sinuous in 

 front for the passage of the byssus. Beaks pointed, terminal or 

 nearly so. Surface smooth or 

 marked with concentric striae. 

 Hinge without teeth. Ligament 

 area broad, and grooved parallel 

 with hinge line. Deep anterior 

 adductor scar under the beak. 

 Siluric-Permic. 



245. M. sancti-ludovici Worth- 

 en. (Fig. 600.) Mississippic. 

 Subquadrate, oblique. Hinge 



straight and equalling the great- 

 est width of the shell below. 

 Margin of shell regularly though 

 narrowly rounded at base, be- 

 coming broadly and shallowly 

 concave towards the hinge line 

 anteriorly and posteriorly. Beak 

 of left valve pointed, curving ob- 

 liquely forward. Shell crossed 

 by strong and regular concentric laminae. Average length of 

 hinge line, | inch ; length of shell at right angles to hinge, 1 inch. 

 Keokuk : Illinois, Kentucky, Missouri. 



246. M. keokuk Worthen. Mississippic. 

 Distinguished from M. sancti-ludovici by its larger size (length 



of hinge, 1.25 inches; length of shell at right 

 angles to hinge, 1.6 inches ; greatest length from 

 umbo forward, 2 inches), greater convexity of 

 body of shell and consequent greater promi- 

 nence of the posterior wing. Umbo stronger 

 and projecting above the hinge line. Anterior 

 Fig. 600. Myalina side of shell more nearly truncate. 

 sancti-ludovici. (Pal. Keokuk : Illinois, Indiana, Iowa, Missouri, 

 '' *' Arizona (Escabrosa) ; also Colorado (Ouray). 



247. M. angulata Meek and Worthen. Mississippic. 

 Differs from M. permiana in its larger size and concave hinge 



Fig. 599. Halobia lommeli ; a slab 

 with numerous individuals. (After Hall 

 and Whitfield.) 



