PLATE LI— Continuetl. 



EUMETRIA VERA, VMr. COSTATA, Hull. 



Fig. 27. The cardinal portion of a pedicle-valve ; showing the completely coalesced deltidial jilates. X 3. 

 Fig. 28. A view of a pedicle-valve from which the shell has been partially exfoliated ; showing the faintly 

 defined muscular area. 



Figs. 31, 32. Dorsal and profile views of the same specimen. 



Chester limestone. Crittenden, county, Kentucky. 



Fig. 29. A dorsal view of a specimen from which the shell has been exfoliated, exposing the elongate, 

 narrow muscular impression of the bi-iichial valve. 



Fig. 30. The upper half of a preparation of the brachidium ; showing in the solid opaque matrix the 

 attachment of the crura to the primary lamelhi;, and the bifurcate extremity of the loop. X 2. 

 Chester limestone. Chester, lUhioii. 



Fig. 33. The interior of the umbonal portion of a brachial valve ; showing the posterior horns of the hinge- 

 plate, the concave median plate and the elongate, narrow dental sockets. The crnral jilates 

 and their pi-ocesses have been lost. X 3. 



Chester group. Crittenden county, Kentucky. 



EuMETRiA Verneuiliana, Hall. 



Figs. 34, 3.5. Dorsal and profile views of a specimen with coarse surface plications. 

 St. Louis group. 8pergen Hill, Tndiana. 



EuMETRIA VERA, Hall. 



Fig. 36. The umbonal portion of an old shell enlarg-ed (o show the thickening of the coalesced deltidial 

 plates which have become conspicuously protuberant. This thickening has been accompanied 

 by a similar growth on the brachial valve which has rendered the flattened cardinal expansion 

 very prominent, as seen (m the right of the Ijeak. The growth of the bi-achial valve has been 

 somewhat unsymmetrical. X 2 



Fig. 37. The umbonal portion of a specimen which has been broken longitudinally nearly in the median 

 axis. On the upper jiortion is exposed the snrfac'e of the more distant of the two ciural plates, 

 flattened below by the transverse concave plate and the upward extension of the nearer of the 

 crural plates. The outer shell is retained about the beak of the pedicle-valve. X 3^. 

 Chester group. Crittenden county. Kentucky. 

 The distinction between the thi'ee forms of Edmetria here represented is one not easy to carry out 

 with an abundance of material. Eumetria Vimeuiliana was founded upon the small, very 

 finely plicated shells from the white limestones at Spergen Hill, Ind., but it was suggested in 

 the original description that the larger shells occuriing in a silicified condition at the same 

 locality and elsewhere, are of the same species. E. vera was based upon specimens of about 

 the same size as the latter, with a somewhat coarser plication, derived fi'om the Kaskaskia 

 (Chester) limestone at Chester, Illinois, and E. vera, var. i-ofitata on larger and more coai'sely 

 plicated shells from the same locality. It is very fi-equently difficult, notwithstanding the slight 

 differences in geological horizon, to distinguish the larger form of E. Verneaitiana from the typ- 

 ical form of E. vera, while a distinction between the two forms of E. Verneuiliana occuring at 

 Spergen Hill is often moi-e readily made. 



Genus ACAMBONA, White. 



Page 119. 

 AcAMBONA ? OSAGENSIS, Su'allow. 



Fig. 38. A dorsal view of an imperfect specimen of the Retzia Osageiuiis, Swallow, which will probably 

 prove to belong to this genus. 



Fig. 39. A portion of the surface of the shell enlarged. The lower part of the figure represents the puncta- 

 tions of the outer surface', where it has lieen exposed and somewhat weathered ; above is the 

 surface of one of the inner layers coveied with fine pustules. The plications are much more 

 distinctly defined on the inner layers, but they are not obsolete on the outer layer as here 

 represented. X 5. 



Choteau limestone. Pike county, Missouri. 



AcAMBONA PRIMA, White. 



Figs. 40, 41. Dorsal and profile views of an incomplete specimen which is i-egarded as lielonging to this 

 species. 



Biirlington limestone. Burlington, Iowa. 



