BRACHIOPODA. 277 



1875. Strophomena, Wiiiru. Geojri'uphical and Geolojrical Kxploi-alidn West lOOtli Mcriiliaii, p. S5, 



pi. V, fig. 5. 



1S77. Strop/Mmeim, Hall and Whitfield. Geol. Expl. 4flth Parallel, vol. iv, p. 2r)3, pi. iv, fig'. 4. 



1877. Plectamhnmteji, D.\ll. Bnll. No. 8, U. S. Nat. Mnsenm, p. SC. 



1879. Strophimiena, Hall. Twenty-eig-hth Repf. N.Y. State Mns. Nat. Hi.st,, ji. 151, pi. xxii, tigs 4-10. 



1SS2. Strophomena, Hall. Eleventh Rept. Indiana State GeologiKt, p. 288, pi. xxii, figs 4-10. 



1883. Strophovuna, Hall. Rept. N. Y. State Geologist for 1S82, pi. xxxviii, figs. 12-31. 



1884. Strophomena, Walcott. Palaeontology Eureka District, p. 118. 



1887. Plectambonites, QEhlert. Fischer's Manuel de Conchyliologie ; Brachiojioda, p. 1285. 



1889. StropliomeiM, Bekcher and Clarke. Mem. N. Y. State Mus. Nat. Hist., vol. i. No. 1, p. IS, iil. ii, 



figs. 1-13. 

 1889. Strophomena, Nettblboth. Kentucky Fossil Shells, p. l.'iO, pi. xviii, figs. 1-3. 



Diagnosis : 



" Testa subgequivalvis, aequilatera, complanata, margine compresso flexo. 

 " Margo cardinalis transversalis, rectilineus, latissimus, foramine destitutu^. 

 " Valva altera deiitibus 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 stria3, which, in well preserved specimens, are crenulated by finer 

 concentric strise. 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 whicli 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 gi'oove 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 consjiicuous, generally supported by lamellae 



