142 University Geological Survey of Kansas. 



from the beaks to the postero-basal extremity, 1.50 inches ; 

 breadth, 0.65 inch ; convexity, about 0.44 inch." 



Range and distribution : Upper Coal Measures ; Topeka. 



This species can be distinguished at once from M. subquadrata 

 by the small angle at the beaks. It differs markedly from M. 

 swallovi in its attenuate beak, and the fact that the beak is not 

 recurved and the umbonal ridge is more nearly straight. 



Myalina congeneris. Plate XVI, figs. 2, 2b. 



Myalina congeneris Walcott, Pal. Eureka Dist, Mon. U. S. Geol. Surv. 

 No. 8, p. 237, pi. xix, f. 6, pi. xxn, f. 10. 



Walcott's description : " Shell oblong, the height being twice 

 the width even in the broad examples ; hinge line straight and 

 ranging at an angle of 60° with the nearly straight anterior 

 margin ; base narrow, but rather broadly rounded, posterior 

 very broadly rounded, curving slightly inwards towards the 

 cardinal line. General surface of the valves strongly convex 

 towards the front border, and beaks becoming more depressed 

 on the posterior portion. Surface of both valves marked by 

 slightly imbricated lamellae of growth." 



Range and distribution : Upper Coal Measures ; Topeka. 



Walcott does not mention the nature of the hinge line in his 

 description. In one of our specimens the beak is exposed, 

 showing it to be acute, extending a little beyond the hinge. 

 The hinge line is similar to that of M. sivallovi in having a sin- 

 gle groove, but there is no swell in front of the beak. Our 

 specimens agree more closely with the narrow form as figured 

 on Plate XIX. 



Our Kansas forms are from nearly the same horizon as M. 

 perattenuata, to which they are most closely related, but differ 

 in having their anterior and posterior margins parallel, and, as 

 a consequence, are not trigonal in outline and the anterior 

 margin is straight, while that of M. perattenuata is concave. 



