BRACH10P0DA. 
277 
1875. Strophomena, White. Geographical and Geological Exploration West 100th Meridian, p. 85, 
pi. v, fig. 5. 
1877. /Strophomena , Hall and Whitfield. Geol. Expl. 40th Parallel, vol. iv, p. 253, pi. iv, fig. 4. 
1877. Plectambonites, Dall. Bull. No. 8, U. S. Nat. Museum, p. 56. 
1879. Strophomena, Hall. Twenty-eighth Rept. N.Y. State Mus. Nat. Hist., p. 151, pi. xxii, figs. 4-10. 
1882. Strophomena, Hall. Eleventh Rept. Indiana State Geologist, p. 288, pi. xxii, figs. 4-10. 
1883. Strophomena, Hall. Rept. N. Y. State Geologist for 1882, pi. xxxviii, figs. 12-31. 
1884. Strophomena, Walcott. Paleontology Eureka District, p. 118. 
1887. Plectambonites, CEhlbrt. Fischer’s Manuel de Conchyliologie ; Brachiopoda, p. 1285. 
1889. Strophomena, Beecher and Clarke. Mem. N. Y. State Mus. Nat. Hist., vol. i, No. 1, p. 18, pi. ii, 
figs. 1-13. 
1889. Strophomena, Nettelroth. Kentucky Fossil Shells, p. 150, pi. xviii, figs. 1-3. 
Diagnosis : 
“ Testa subsequivalvis, sequilatera, complanata, margine compresso flexo. 
“ Margo cardinalis transversalis, rectilineus, latissimus, foramine destitutus. 
“ Valva altera dentibus cardinalibus duobus obtusis.” —Dalman, op, cit., p. 94. 
Shells plano-convex when young, concavo-convex at maturity ; convexity 
normal. Surface covered by conspicuous concentric corrugations or wrinkles 
over the flatter portions of the valves. Where these cease the surface is more 
or less abruptly and often rectangularly deflected, forming a conspicuous an¬ 
terior slope. The whole exterior is covered with fine, even, radiating, thread¬ 
like, tubular striae, which, in well preserved specimens, are crenulated by finer 
concentric striae. Outline transversely subquadrate or semioval. Hinge-line 
straight, its length making the greatest diameter of the shell; extremities often 
subauriculate. Cardinal area narrow, slightly wider on the pedicle-valve, not 
denticulate. In the pedicle-valve, the delthyrium is covered by a convex 
deltidium, perforated at the apex by a foramen which is -closed at maturity or 
encroaches upon the apex of the valve. This deltidium is most conspicuously 
developed in early stages of growth, then having the form of a tube or 
sheath, which character becomes obliterated as maturity approaches, by the 
increase in the size of the cardinal process of the opposite valve, and the cal¬ 
losity formed about its base. In adult shells the foramen has become enclosed 
by the substance of the shell, its external opening being an oblique groove in 
front of the apex of the valve, and its inner aperture appearing in front of the 
pedicle-scar. Not infrequently the passage is closed at maturity. The teeth 
are very divergent and quite conspicuous, generally supported by lamellse 
