PLATE X—Continued. 
Derbya robust a, Hall. 
Fig. 12. The exterior of a compressed brachial valve, in which the radial striae of the shell are strongly 
developed. 
Fig. 13. An enlargement of the cardinal process from another specimen. 
Figs. 14 [17 in error], 15. Profile and brachial views of a large individual; showing the external charac¬ 
ters of the species. 
Fig. 16. The interior of the cardinal portion of the pedicle-valve ; showing the character of the muscular 
scars, the strong but short median septum, projecting teeth and the convex deltidium. The 
projecting tooth on the right-hand side of the figure hag become double from abnormal growth. 
Fig. 17. The exterior of a pedicle-valve ; showing the character of the striae, which differ in some degree 
from their uniformity shown in the specimen represented in figs. 14 and 15. 
Coal Measures. St. Clair county, Illinois. 
For other illustrations, see Palaeontology of Eastern Nebraska, plates v and viii. 
Genus MEEKELLA, White and St. John. 
Page 26t. 
Meekella striatocostata, Cox. 
Fig. 18. Profile of a specimen retaining both valves; showing the great elevation of the.cardinal area of 
the pedicle-valve. 
Fig. 19. Cardinal view of a larger individual; showing the high area, narrow deltidium, and the bases of 
the crural plates in the brachial valve. 
Fig. 20. A view of the pedicle-valve of the same specimen. 
Fig. 21. A cardinal view of a specimen from which the apex of the pedicle-valve has been removed to 
show the dental lamellae. 
Fig. 22. An enlargement of the cardinal process ; showing the extreme elevation of the lobes and the cru¬ 
ral plates. The central portion of the process is incorrectly represented as having three instead 
of two lobes. 
Fig. 28. A longitudinal section of the specimen represented in fig. 19 ; showing the extent of the dental 
lamellae and crural plates, to the latter of which one of the crura is attached. 
Coal Measures. Near Winterset, Iowa. 
For other illustrations, see Transactions of the Chicago Academy of Natural Sciences, Volume I, 
pp. 120, 121; and Palaeontology of Eastern Nebraska, plate v. 
