PLATE LXXXI. 



(Figures 1-3, 5--21, 2«. ■»)-« liy E Kmmons ; 4 by G. B Simpson ; i2-.'5,27,'28 by 1{. P. Whitfikld ) 



Genus DIELASMA, King. 



Page "U. 



DlELASMA TURQIDA, HfXll. 



Figs. 1-3. Dorsal, ventral and profile views of an averafje specimen with median sinus on both valves. 

 Fig. 4. Profile of an old shell with highly ventricose valves. 



St. Louis group. WanMngtoa county, Indiana. 

 Figs. 5, 6. Front and dorsal views of a .small shell having the median sinus very broad. X 2. 

 Figs. 7, 8 Front and doreal views of a specimen having a single plication in the sinus of the pedicle-valve 

 and a corre8pon<iing groove on the opposite valve. X li- 



Chester group. Spenoer ciinniij, Indiann 



D1EI.A8MA BUULIXCXONEN.SIS, Wliite. 



Figs. 9- 1 1 Dorsal, profile and ventral views of a small shell. 



Chert of the Burlington limestone. Loiti-siatia, Mixxowri. 



blELASMA FORMOSA, Hilll. 



Figs. 12, 13. Profile and dorsal views of a small internal cast. 



Fig. 14. Profile of a shell of atxiiit the same size, but with more gibbous valves. 



Fig. 15. Doi-sal view of an internal cjust ; showing that in these small shells the muscular platform of the 



brachial valve is not clearly defined. 

 St. Louis gi-oup. 

 Fig. 16. A cast of the caMinal portion of the pedicle-\ alve, enlarged to show the cavity left by the enfolded 



margin of the i)edicle-pa.«sage, the dental lamellae and deltidial jilates. X 2. 

 Fig. 17. An internal cAst of the brachial valve ; showing the impression left by the base of the muscular 



platform, and the genital markings about the mu.scular area. 

 Figs. 18, 19. Profile and dorsal views of a specimen of medium size. 



8t. I.,ouiH group. 

 Fig. 20. Dorsal view of a large individual retaining the shell. 

 Fig. 21. Dorsal view of a specimen cut to expose the loop. 

 Figs. 22, 23. Ventral and iloi-sal views of the loop of the same specimen enlarged. As is usual in these 



shells the recurvetl lamella is not i>ei-fectly retained 

 Fig 24. Dorsal view of a large internal cast ; showing a portion of the base of the muscular platform, and 



divergent, probably vascular impressions over the adjoining surface. 

 Figs 25, 26. Pi-ofile and dorsal views of a characteristic and somewhat gibbous example. 

 St. Louis group. 



DlELASMA ROWLEVI, WoHllCU. 



Figs. 27, 28. Profile and doi-sal views of an internal c-ast, referred with hesitation to this species; showing 

 the muscular platform of the bra<;hial valve, and the cavities of the dental lamellae. 

 Choteau limestone. Orai/dtm Uprings, Mixsi/uri. 



DlELASMA BOV1DEN8, Moitoil 



Figs. 29, 30. Dorsal and pi-ofite views of a rather gibbous sjiecimen ; showing the contour of the valves and 

 the oblicjue foramen The c<jni!entrii; surface lines appear to be to some extent color-markings, 

 and not infre(piently faint traces of a radial coloration ai'e discernible in the shells. This 

 specimen is of about the average size of the species as it occurs at this locality. 

 Upper Carboniferous. Kansas Oity.^Missouri. 



