8 



Indiana University Studies 



of the specimens at hand. The anterior margin is gently concave below the 

 beaks, which are sharp, but not recurved. A large portion of the posterior 

 part of the shell is nearly flat. There is a slight sinus below the posterior 

 extremity of the hinge as in M. subquadrata, which makes the shell slightly 

 lobate posteriorly. Toward the ventral region the anterior and posterior 

 margins are at right angles to the hinge. 



When the size of the shell is taken into account, the height of the hinge 

 area is very small, which is in accord with the thinness of the shell. The 

 right valves are relatively much shallower within and flatter in general 

 appearance than the right valves of M. subquadrata. The plate in the right 

 v^alve at the front end of the hinge is strongly developed. 



Length of hinge 



Width of hinge area in center. 

 Number of lines on hinge area 



Depth of valve cavity 



Convexity of valve 



Depth of valve 



M. shannoni M. subquadrata. 



70. 35.5 

 5. 7.25 



7. 10. 

 8.5 7.5 



18.5 11.5 



about 105. 56. 



Another specimen from a different horizon and locality gave a length 

 of hinge of 50 mm., and a height of 80. The hinge is as long as the shell at 

 any point below it. 



Willard shales(?); near Foster, Oklahoma. 



From the above measurements it is at once apparent that this is a very 

 thin-shelled species of large size, with a narrow hinge plate and slight con- 

 vexity when compared with M. subquadrata Shumard with which it is asso- 

 ciated. The manner in which the growth marks approach the hinge separates 

 it at once from M. ampla Meek. In none of the specimens from Foster was a 

 complete shell seen, and on examination it was found that the lower portion 

 of the shell was almost paper thin, in striking contrast to M. subquadrata 

 Shumard. 



Myaliiia girtyi n.sp. 



The shell is fairly large for the genus, transversely quite elongate and 

 rather narrow. The hinge is nearly straight and shorter than the length of 

 the shell below it. The beak is well produced, sharp at the point, but not 

 recurved. The hinge is straight and the ligamental area is wide. The ventral 

 part of the shell is ovate in outline. The anterior margin is somewhat 

 concave and joins the beak, making a 45° angle with the hinge. The posterior 

 part of the shell is convex thruout, rounding obliquely forward to meet the 

 hinge. The right valve, as in some other species of the genus, is flatter and 

 less distinctively ornamented than the left. The left valve is ornamented 

 by lines of growth, regularly and rather widely spaced. The main axis of the 

 shell makes an angle of 100° with the hinge. The posterior extremity of the 

 hinge is rounded off somewhat, since the later plates project farther to the 

 rear than the earlier ones, as the shell increases in size, completing the 

 rounded appearance of the posterior region. 



The interior of the shell is marked by a wide ligamental area, with 10 

 to 14 grooves and ridges, and the sharp projection of the plate beneath 



