164 
PALEONTOLOGY OF NEW YORK. 
1863. Atrypa, Billings. Geology of Canada, p. 315, fig. 321; p. 318, fig. 335; p. 384, fig. 416. 
1865. Atrypa, Davidson. British Devonian Brachiopoda, jip. 53-59, pi. x, figs. 3-13; pi. xi, figs. 1-12. 
1865. Atrypa, Shalbr. Bull. Mus. Comparative Zoology, No. 4, p. 68. 
1867. Atrypa. Davidson, British Silurian Brachiopoda, pp. 130-136, pi. xiv, figs. 1-22; pi. xv, 
figs. 1-8. 
1867. Atrypa, ‘Whitfield. Twentieth Kept. N. Y. State Cah. Nat. Hist., p. 141. 
1867. Atrypa, Hall. Palieontology of N. Y., vol. iv, pp. 312-327, plates li, lii, figs. 1-12 ; pis. liii, liiiA. 
1868. Atrypa, Msek and Woethen. Geol. Survey of Illinois, vol. iii, p. 430, pi. xiii, fig. 7. 
1868. Trematoapira, McChbsney. Trans. Chicago Acad. Sci., vol. i, p. 32, pi. vii, fig. 3. 
1868. Atrypa, Meek. Trans. Chicago Acad. Sci., vol. i, jiii. 96, 97, pi. xiii, figs. 12, 13. 
1868. AtryiJa, Meek and Worthen. Geol Survey of Illinois, vol. iii, p. 432, pi. xiii, fig. 11. 
1872. Atrypa, Hall and Whitfield. Twenty-fourth Kept. N. Y. State Mus. Nat. Hist., pp. 197-199. 
1875. Atrypa, Hall and Whitfield. Palaeontology of Ohio, vol. ii, p. 133, pi. vii, figs. 12-14. 
1877. Atrypa, Meek. King’s U. S. Geol. Expl. Fortieth Parallel, vol. iv, p. 38, pi. i, fig. 7; pi. iii, fig. 6. 
1878. Atrypa, Etheridge. Quart. Jour. Geol. Society London, vol. xxxiv, p. 596. 
1879. Atrypa, Hall. Tw'enty-eighth Kept. N. Y. State Mus. Nat. Hist., p. 162, jd. xxv, figs. 44-47. 
1880. Atrypa, White. Second Ann. Kept. Indiana Bureau of Statistics and Geol., p. 502, pi. v, figs. 7-9. 
1881. Atrypa, White. Tenth Kept. State Geologist of Indiana, p. 134, pi. v, figs. 7-9. 
1882. Atrypa, Hall. Eleventh Kept. State Geologist of Indiana, p. 304, pi. xxv, figs. 44-47. 
1882. Atrypa, Whitfield. Geology of Wisconsin, vol iv, 333, jih xxvi, figs. 5-8. 
1884. Atrypa, Walcott. Monogr. U. S. Geol. Survey, vol. viii, p. 150, pi. xiv, figs. 4, 6. 
1885. Atrypa, Foerste. Bull. Denison University, vol. i, ji. 90, jd. xiii, fig. 9. 
1889. Atrypa, Beecher and Clarke. Memoirs N. Y. State Museum, vol. i, p. 51, jd. iv, figs. 12-20. 
1889. Atrypa, Nettelroth. Kentucky Fossil Shells, pp. 88-92, pi. xiv, figs. 1-23; pi. xv, fig. 1 ; 
jd. xxxii, figs. 5-8, 44-47, 64-66. 
1890. Atrypa, Foerstb. Proc. Boston Society Nat. Hist., vol. xxiv, p^i. 314-316, jd. vi, figs. 8, 9. 
Diagnosis. Shells subcircular or longitudinally suboval in outline. Gibbous, 
strongly inequivalve. Hinge-line short, straight; cardinal extremities rounded. 
Beaks not prominent. 
Pedicle-valve the smaller; convex in the umbonal region, but depressed and 
often deeply sinuate anteriorly. Beak small, usually incurved in advanced 
growth-stages, concealing the foramen and deltidium. The foramen is triangu¬ 
lar in young shells, extending to the hinge-line, but becoming gradually closed 
by the growth of deltidial plates, and at maturity is circular and apical, en¬ 
croaching slightly on the substance of the valve. The plates of the deltidium 
are not coalesced along the median suture. On the interior the umbonal cavity 
is short but very broad. The teeth are large, widely separated and doubly 
grooved, first by an oblique furrow at the base, into which is fitted a crenulated 
ridge of the other valve, then by a short longitudinal depression on the summit; 
the tooth is doubly curved and reflected, making the articulation of the valves 
very firm. These teeth arise from the inner surface of the lateral slopes of the 
