77 



upper posterior edge sometimes curving a little outwards 

 before joining the hinge above, so as to form a very small 

 more or less angular posterior wing ; though it often makes 

 no curve, but meets the hinge line at a very obtuse angle. 

 Lower valve more compressed or even concave, but having 

 much the outline of the other, excepting that its anterior 

 border, just below the hinge, is provided with a rather 

 large, profound sinus for the passage of the pedal muscle, 

 and extending back parallel to the hinge to a point nearly 

 under the beaks. 



Surface ornamented by distinct radiating costae usually 

 less than the spaces between, and provided with regularly 

 disposed prominent spinelike vaulted processes, formed by 

 the projecting lamellae of growth. Between each two of 

 these larger costae, from one to three or four much smaller, 

 elevated, threadlike, radiating lines are seen ; while much 

 finer concentric striae mark the surface in the other direc- 

 tion. Length 1*40 inches; height 1*42 inches; breadth 

 about 0'40 inch. 



This species is very closely related to M. Speluncaria, 

 (Schot.) from some varieties of w T hich, as figured by Mr. King 

 in his work on the Permian Fossils of England, it will not 

 be always easy to distinguish it. Our shell however, ap- 

 pears never to have the beak of the larger valve so elevated, 

 nor the posterior side so distinctly lobed as we sometimes 

 see in that species. We have named it in honor of Major 

 F. Hawn. 



Locality. — Near mouth Smoky Hill fork, Kansas river, and 

 near Helena, Kansas. 



Myalina (Mytelus) per attend" at a. 

 Shell thin and fragile, elongate, arcuate, and very attenu- 

 ate towards the beaks ; valves much compressed posteriorly 

 and above, convex, and subangular near the beaks, and along 

 the ant ero -ventral side. Beaks terminal, nearly straight, 

 or very slightly declining, pointed and directed forward. 



