BRACHIOPODA. 
255 
Genus RENSSEL^RIA, Hall. 1859. 
PLAIES LXXV, LXXVI, LXXVIl. 
1832. Tereh-atula, Eaton. Geological Text-book, p. 45. 
1839. Atrypa, Conead. Geological Survey of N. Y.; Third Ann. Kept. Pal. Dept., p. 65. 
1842. Ati'ypa, Conead. Journal Acad. Nat. Sci. Philadelphia, vol. viii, p. 266, pi. xvi, fig. 17. 
1842. Atrypa, Vanuxem. Geology of N. Y. ; Kept. Third Dist., p. 123, fig. 2. 
1843. Ati-ypa, Hall. Geology of N. Y.; Kept. Fourth Dist., p. 148, fig. 2. 
1843. Pentamerus, Castelnau. Essai aur le Syst. Silurien de I’Amer. Septen., p. 38, jil. xv, figs. 1, 2. 
1844. Tei-ebratula, F. Roemer. Das rhein. Uebei-gangsgebirge, p. 68, jd. i, figs. 6 a, b. 
1857. Meganteris, Hall. Tenth Kept. N. Y. State Cab. Nat. Hist., pp. 97-99, 101-103. 
1858. Megantei-is, Rogers. Geology of Pennsylvania, vol. ii, pt. ii, p. 826, fig. 649. 
1859. Remselmria, Hall (partim). Twelfth Rept. N. Y. State Cab. Nat. Hist., pp. 39-41. 
1859. Rensselmria, Hall. Palaeontology of New York, vol. iii, pp. 254-256, 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., pi. xi, fig. 14. 
1864. .* Rensselwria, Davidson. Monogr. British Devonian Brachiopoda, p. 10, pi. iv, figs. 5-7. 
1870. RensselCBria, Dall. American Journal of Conchology, vol. vi, p. 105. 
1874. Rensselmna, Billings. Palaeozoic Fossils, vol. ii, p. 41, pi. iii, figs. 7, 10. 
1876. Rensselwria, F. Roemer. Lethaea Palaeozoica, pi. xxiii, fig. 5. 
1882. ? Rensseloeria, Davidson. Suppl. Devonian Brachiopoda, 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 oftbn strongly inflectted. 
Pedicle-valve with full, 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- 
