PLATE 50. 



Pago 

 Figs. 1 - 12. Meristella doris 80:j 



I. A dorsal view of a small specimen, from the Scholiarie grit. 



'2. A dorsal view of a small individual. This, and the following figures, are of specimens from 



the Comiferons limestone. 

 8. 4. Dorsal and ventral views of an unsymmetrical specimen, of medium size. 



6, 6. Dorsal and profile views of a specimen of medium size and perfect form. 



7. Ventral view of a large individual, whicli is .somewhat imperfect at tlie beak. 

 8-10. Dorsal, profile and (rout views of a large individual. 



II. The dorsal valve of a large individual. 



12. Cardinal view of a cast, showing the muscular impressions and the cavities made by the 

 removal of the dental lamellae and median septum. 



Figs. 13-17. Meristella rostrata "M~ 



18- 16. Dorsal, ventral, profile and front views of a specimen. 



17. Dorsal view of a specimen, having a somewhat ditlerent form from the above. 



Figs. 18-35. Meeistella (Pentagonia) unisulcata 30i) 



18-20. Dorsal, ventral and cardinal views of a small si^cimen, presenting the ordinary charac- 

 ters of the species. 



21 . Dorsal view of a small individual, having a less extended hinge line, giving the shell a more 



triangular form. 



22, 23. Dorsal and ventral views of a specimen of medium size. 



24. A cardinal view of a somewhat larger individual, showing a deep siuus in the mesial fold. 



26-29. Dorsal, ventral, front, cardinal and profile views of a large but not very gibbous specimen. 



30-32 Ventral, front and cardinal views of a narrow gibbous form, from the Falls of the Ohio. 



83, 34. Interiors of the dorsal and ventral valves, .showing the hinge structure and muscular im- 

 pressions. From the Falls of the Ohio, communicated by Major S. S. Lyon. 



35. A cardinal view of a specimen, from the lliimilton shale.s of New York, showing a second 

 fold on the cardinal margin of the dorsal valve. 



