200 Trans. Acad. Sci. of St. Louis. 



traversed by a few imbricating lines of growth, which 

 increase in number near the border in the older specimens. 

 Pedicle valve with a narrow, faintly impressed sinus extend- 

 ing from the beak along the shell, corresponding to the inner 

 septum, which gradually expands into a broader and deeper 

 depression, and, with a corresponding elevation in the oppo- 

 site valve at the margin, gives it a considerable sinuosity in 

 front. Beak short, acute, and slightly incurved. A minute 

 round foramen just beneath the apex. False area small, con- 

 cave. Longitudinal septum not extending beneath the beak, 

 but ending about even with the cardinal teeth. Brachial 

 valve more gibbous than the pedicle, umbo prominent, longi- 

 tudinal septum extending the full length of the shell, but be- 

 coming indistinct at the front margin. A narrow, scarcely 

 perceptible impression extends along the back, opposite the 

 septum. The spatulate portion of the cardinal process short, 

 and bending slightly upward, to correspond to the under side 

 of the concave area, beneath which it passes at nearly a right 

 angle to the basal portion. The crura, being very small, 

 serve to give sharpness to the angle, and also, by slight lateral 

 projection in front of the cardinal teeth, give security to the 

 hinge. Length of shell from 8 mm. to 10 mm., breadth 

 from 10 mm. to 12 mm." 



MOLLUSCA. 



GASTEROPODA. 

 Worthenia mississippiensis ( W. and W. ). 



PL XX. f. 12. 



Pleiirotomariamississippiensts, Bull. U. 8. G. S. 153: 457. 



Original description. " Shell rather above a medium size, 

 spire elevated, composed of five or six volutions ; the height 

 ii little greater than the diameter of the base. Volutions 

 flattened on the upper side, the plane extending from the 

 suture to the middle of the whorl, regularly rounded on the 

 inner side. Periphery marked by a revolving band, which on 

 the outer volution is an eighth of an inch in breadth, prom- 

 inent at the margins and depressed in the center. Volutions 



