140 University Geological Survey of Kansas. 



Upper Missouri as M. subquadrata is, as we then suspected, a 

 distinct species, differing in having the posterior margin round- 

 ing forward into the hinge above, and not sinuous or meeting 

 the hinge at right angles, as in M. subquadrata. It will have to 

 take the name M . ampla, suggested by us for it, in case it should 

 be found distinct." 



I have collected a large specimen of this species near Topeka 

 with both valves present, but separated. 



Myalina kansasensis. Plate XVI, fig. 11. 



Myalina kansasensis Shuniard, Trans. St. L. Acad. Sci., i, p. 213, (1858); 

 Keyes, Geol. Surv. Mo., v, p. 117, pi. xliii, f. 5, (1894). 



Original description: "Shell sub-rhomboidal, sub-inequi- 

 valve, inequilateral, gibbous, the left valve more gibbous than 

 the right ; height about double the length ; in young specimens 

 the greatest length is at the cardinal border, but in the adult 

 toward the pallial margin ; cardinal margin oblique, slightly 

 arched, and forming with the posterior border an angle of 

 about 120° ; posterior margin rather strongly arched in adult 

 specimens, and very gently rounded in the young ; pallial mar- 

 gin rounded ; buccal margin concave ; umbones very prominent 

 anteriorly, and declining with a moderate slope to the posterior 

 margin ; anterior slope very abrupt ; beaks terminal, attenu- 

 ated, directed obliquely forward, incurved ; surface with strong, 

 imbricating, sub-equidistant, concentric lamellae, whose free 

 edges are often irregularly crenate ; lamellae most prominent 

 on the left valve. The ligament face is broad, marked with 

 equidistant, close, deeply impressed lines parallel to the cardi- 

 nal edge, the number varying with the age of the shell; be- 

 neath these is a rather broad, smooth space, which is continuous 

 with a similar space extending from the pallial region. Each 

 valve exhibits a singular muscular impression, which is large, 

 ovate, and situated towards the posterior margin. Height, 2-J- 

 inches; length, 1.17 inches; thickness, 0.88 inch. 



Range and distribution : Upper Coal Measures ; Argentine, 

 Cowley county, Neosho river near Council Grove. 



This species may be easily recognized by the arched lamellae 



