PLATE X— Continued. 



Derbya robusta, Hall. 



Fi^. 12. The exterior of a compressed biacLial valve, in wliitb the ladial stria? of the .-sliell are strongly 

 lieveloped. 



Fig. 13. An enlargement of the cardinal process from another specimen. 



Figs. 14 117 in error], 15. Profile and brachial views of a larjje individual; showany the external charac- 

 ters of the species. 



Pig. 16. The interior of the caniinal poi'tlon of the pedicle-valve ; showing the character of the niuscnlai- 

 scai-s, the strong but short median septum, pi-ojecting teeth and the convex deltidium. The 

 projecting tooth on the right-hand side of the tigure has become double from abnormal growth. 



Fig. 17. The extei-ior of a pedicle-valve ; showing the character of the striie, which ditfer in some degree 

 from their uniformity shown in the specimen represented in tigs. 14 ami 15. 

 Coal Measures. St. Clair comity. Illivois. 

 For other illustrations, see Paleontology of Eastei-n Nebraska, plates v and viii. 



Genus MEEKELLA, White and St. ,Iohn. 



Page 264. 



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. Cardin.al view of a larger individual ; showing the high ai'ea, narrow deltidium. ami the ba.ses of 



the criu'al plates in the brachial vahe. 

 Fig. 20. A view of the pedicle-valve of the same .sjiecimen. 

 Fig. 21. A cardinal view of a specimen from which the apex of the pedit'le-valve has been removed to 



show the dental lamellae. 

 Fig. 22. An eidargement 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 a.« having three instead 



of two lobes. 

 Fig. 23. A longitudinal section of the specimen represented in fig. 19 ; showing the extent of the ilental 



lamellas and cniral plates, to the latter of which one of the crura is attached. 

 Coal Measui'es. Near Winierset. Iowa. 

 For other illustrations, see Transactions of the Chicago Academy of Natui'al Sciences, Volume I, 



pp. 120, 121 ; anil Palaeontology of Eastern Nebraska, plate v. 



