DIELASMA 259 



depressed, radiating costie, about three of which occupy the space of one 

 millimeter; the concentric lines of growth are more or less obscure in 

 smaller individuals, becoming somewhat stronger and more crowded 

 towards the outer margin of large individuals. Shell structure finely 

 and closely punctate. 



Remarks. This species is rather common in the Chouteau limestone 

 of centra] Missouri. Hall and Clarke illustrated a large, typical example 

 of the species as Dielasma formosa, but it may be easily distinguished from 

 that species by the more depressed convex valves which are much more 

 regularly convex transversely. Internally the two species are also dis- 

 tinct, though both possess the essential characters of Dielasma; in D. 

 chouteauensis the muscular platform of the brachial valve becomes elevated 

 free from the inner surface of the valve anteriorly, while in D. formosa 

 the median contact is retained throughout. The fine, radiating costce have 

 been observed on only a few examples; it is possible that they were 

 uniformly present upon the living shells, but have usually been obliterated 

 in the fossils by exfoliation since all the known examples occur in that 

 condition. 



Horizon. Chouteau limestone. 



DIELASMA FERNGLENENSIS Weller 



Plate XXXIII, Figs. 4-5 



1909. Dielasma fernglenensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 318, 

 pi. 14, fig. 7. 



Description. Shell large, subovate in outline, longer than wide, the 

 greatest width a little anterior to the middle. The dimensions of the 

 type specimen, as nearly as can be determined, are : length of pedicle 

 valve 55 mm., length of brachial valve 51.4 mm., width 43 mm., thick- 

 ness 25 mm. 



Pedicle valve moderately convex, the curvature greatest towards the 

 postero-lateral margins, becoming inflected towards the cardinal extrem- 

 ities ; mesial portion of the valve not differentiated from the general curv- 

 ature of the surface ; beak pointed and closely incurved, pierced by a large 

 foramen which encroaches wholly upon the umbonal portion of the valve, 

 the deltidial plates hidden by the incurvature of the beak. Internally the 

 dental lamella? well developed an'd of moderate length. 



Brachial valve probably about equally convex with the pedicle, the 

 curvature strongest towards the postero-lateral margins, rather gentle to 

 the antero-lateral and anterior margins; mesial portion of the valve not 

 differentiated from the general curvature of the surface ; beak pointed and 

 incurved beneath the pseudodeltidium of the opposite valve. Internally 

 the crural lamelke diverge from the beak and are separate from the socket 

 plates, being joined transversely by a concave, muscle-bearing plate as in 



