BRACHIOPODA. 255 



Genus RENSSEL^RIA, Hall. 1859. 



PLATES LXXV, LXXVI, LXXVIl. 



1832. Terehratula, Eaton. Geological Text-book, p. 45. 



1839. Atrypa, Conhad. Geoloffical Survey of N. Y. ; Third Ann. Kept. Pal. Dept., p. 65. 



1842. Atrypa, Conrad. Journal Acad. Nat. Sci. Philadelphia, vol. viii, p. 2G6, pi. xvi, fig. 17. 



1842. Atrypa, Vanuxem. Geology of N. Y. ; Rept. Third Dist., p. 123, fig. 2. 



1843. Atrypa, Hall. Geology of N. Y. j Rept. Fourth Dist., p. 148, tig. 2. 



1843. Pentamerus, Castelnad. Essai sur le Syst. Silurien de I'Amer. Septen., p. 38, pi. xv, figs. 1, 2. 



1844. Terebratida, F. Roemeb. Das I'hein. Uebergangsgebirge, p. 68, pi. i, figs. 6 a, 6. 



1857. Meganterls, Hall Tenth Rept. N. Y. State Cab. Nat. Hist., pp. 97-09, 101-103. 



1858. Megantei-is, Rogers. Geology of Pennsylvania, vol. ii, pt. ii, p. 826, fig. 649. 



1859. Rensselieria, Hall (partini). Twelfth Rept. N. Y. State Cab. Nat. Hist., pp. 39-41. 



1859. Rensselmria, Hall. Palseontology of New York, vol. iii, pp. 254-2')6, 453-457, 461-464, pi. xlv, 



figs. 2-4 ; pis. civ, cv, cviii. 

 1862. Atrypa, (Conrad) Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., i>I. xi, fig. 14. 

 1864. .* Rensselwria, Davidson. Monogr. British Devonian Braehiopoda, p. 10, pi. iv, figs. 5-7. 

 1870. Rejisselwria, Dall. American Journal of Conchology, vol. vi, p. 105. 

 1874. Rensselmrla, Billings. Palseozoic Fossils, vol. ii, p. 41, pi. iii, tigs. 7, 10. 

 1876. Rensselasria, F. Roemee. Lethtea Palaeozoica, pi. xxiii, fig. 5. 

 1882. ? Rensselmria, David.so.v. Suppl. Devonian Braehiopoda, p. 19, pi. i, figs. 20, 20 a. 



Diagnosis. Shell oval or elongate-ovate in outline, subovoid in contour, 

 broadest posteriorly ; valves unequally convex, with a more or less distinct 

 median elevation on both. Cardinal slopes broadly flattened in the typical 

 species. Lateral margins compressed and often strongly intlectted. 



Pedicle-valve with tuU, scarcely salient umbo; beak acute, incurved, with 

 terminal foramen. Beneath and on each side of the beak is a concave, sharply 

 defined space, but no proper cardinal area. Distinct deltidial plates divided by 

 a median suture may be present, but they are usually concealed by incurvature, 

 or atrophied by the encroachment of the umbo of the opposite valve. 



On the interior the teeth are prominent but not thickened, widely divergent 

 and close within the margins of the valve. They are supported by stout dental 

 plates which rest for most of their length upon the bottom of the valve, but 

 are free along their posterior margins. These plates are closely appressed 

 against the lateral walls of the shell and become coalescent therewith in later 

 growth-stages. The muscular impression is restricted to the posterior portion 

 of the valve, covering a very narrow longitudinal area. Between the dental 

 plates at the bottom of the pedicle-cavity lies a narrow median scar, and in front 

 of this a very deep, elongate impression which is sometimes clearly divided 

 into central adductor scars embraced by the diductor impressions. This distinc- 



