PLATS U— ContiDued. 

 EUMETRIA VEKA, Vlir. COSTATA, llilll. 



fig. 27. The cardinal portion of a i>edicle-valve J showing the completely coalesced deltidial plates. X 3. 



Fig. 38. A view of a jieilicle-valve from which the shell has been partially exfoliated ; showing the faintly 

 defined muscular area. 



Figs. 31, 83. Dorsal and profile views of the same specimen. 



Chester limestone. Crittenden county, Kentucky. 



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

 narrow muscular impi-ession of the brachial valve. 



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

 attachment of the crura to the primary lamellie, and thu bifurcate extremity of the loop. X 2. 

 Chester limestone. Chexter, Illinois. 



Kg. 33. The intei-ior of the urabonal portion of a brachial valve ; showing the posterior horns of the hinge- 

 plate, the concave median plate and the elongate, narrow dental sockets. The crural plates 

 and their processes have been lost. X 3. 



Chester group. Crittenden county, Kentucky. 



EuMETRIA VeRNEUILIANA, Hilll. 



Figs. 34, 35. Doi-sal and profile views of a specimen with coarse surface plications. 

 St. Louis group. Spergen Hill, Indiana. 



Eumetkia vera, Hall. 



Fig. 36. The umbonal portion of an old shell enlarged to 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 on the right of the beak. The growth of the brachial valve has been 

 somewhat unsynimeti-ical. X 2 



Pig. 37. The umbonal portion of a specimen which has been broken longitudinally neai-ly in the median 

 axis. On the upper portion is exposed the surface of the more di.slant of the two crural plates, 

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

 crural plates. The outei- shell is retained about the beak of the pedicle-valve. X li. 

 Chester group. Crittenden county, Kentucky. 

 The distinction between the three forms of Eombtria here i-epresented is one not easy to carry out 

 with an abundance of material. Eumetria VtrJinuiliana 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 occurring in a silicified condition at the same 

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

 the same size aa the latter, with a somewhat c^)ar8er plication, derived from the Kaskaskia 

 (Chester) limestone at Chester, Illinois, and E. vera, var. costata on larger and more coarsely 

 plicated shells from the same locality. It is very fiequently difficult, notwithstanding the slight 

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

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

 Spergen Hill is often more readily made. 



Genus ACAMBONA, White. 



Vagc 119. 



Aoambona? Osagensis, Swallow. 



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

 prove to Iwlong to this genus. 



Fig. 89. 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 been 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. 



Aoambona prima, White. 



Figs. 40, 41. Dorsal and profile views of an incomplete specimen which is regarded as belonging to this 

 species. 



Burlington limestone. Burlington, Iowa. 



