154 PALEONTOLOGY OF NEW YORK. 



Gends ZYGOSPIRA, Hall. 1862. 



PLATE LV. 



1847. Alrypa, OrOiis f, Stmocisvia, Hall. Palaeontology of N.Y., vol. i, pp. 140-142, 288, pi. xv, fig. 15 ( 

 pi. xxxiii, figs. 4, 5 ; pi. Ixxix, fig. 5. 



1859. Jthynchonellaf, Hall. Twelfth Rept. N. Y. State Cab. Nat. Hist., p. 66. 



1860. Atrypa, Ukhh. Thirteenth Rept. N. Y. State Cab. Nat. Hist., p. 69. 



1862. Zygotpira, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., p. 164, figs. 1, 2. 



1862. Zygotpira, Billinqs. Canadian Naturalist and Geologist, vol. vii, p. 393. 



1862. Athyris, Billinos. Palieozoic Fossils, vol. i, p. 147, figs. 125-127. 



1863. AUiyris, Rhynchonellat, Billikos. Geology of Canada, p. 168, fig. 152; p. 211, fig. 211 ; p. 212, 



figs. 214-216. 



1864. Stenocixma, Mbbk and Hatobn. Palaeontology of the Upper Missouri, p. 16. 



1866. SliyncJionella, Zygospira, Billinqs. Catalogue Silunan Fossils of Anticosti, pp. 44, 46. 



1867. Zygospira, Hall. Twentieth Rept. N. Y. State Cab. Nat. Hist., p. 267. 



1868. Zygospira, Mbek. Geological Survey of Illinois, vol. iii, p. 377. 



1872. Zygospira, Hall. Twenty-third Rept. N. Y. State Cab. Nat. Hist., pi. xiii, figs. 23-25. 



1873. Zygospira, Mbbk. Paleontology of Ohio, vol. i, pp. 125, 126, pi. xi, figs. 4, 5. 

 1875. Zygoxpira, Millkr. Cincinnati Quarterly Journal of Science, vol. ii, pp. 58, 59. 



1878. Zygospira, U. P. Jambs. The Palffiontologist, No. i, p. 7. 



1879. Zygospira, Ulrich. Journal Cincinnati Soc. Nat. Hist., vol. ii, p. 14, pi. vii, fig. 10. 

 1882. Zygospira, Hall. Eleventh Rept. State Geologist of Indiana, p. 305, pi. xxvii, fig. 7. 



1882. Zygospira, Anazyga, David.son. British Silunan Brachiopoda, Supplement, pp. 122, 128. 



1883. Zygospira, Hall. Transactions of the Albany Institute, vol. x, p. 70. 



1889. Zygospira, Nettblroth. Kentucky Fossil Shells, p. 138, pi. xxxiv, figs. 21-25. 

 1893. Zygospira, 'Wijicbell and Schdcbebt. Geological Survey of Minnesota, vol. iii, pp. 465-469, 

 pi. xxxiv, figs. 42-48. 



Diagnosis. Shells usually small. Outline subcircular or transversely oval. 

 Contour subplano-convex. Surface sharply plicate. Pedicle-valve with a 

 median plicated ridge. Umbo narrow and prominent; beak acute and in- 

 curved. Foramen elongate, rarely apical, enclosed by the deltidial plates. 

 Hinge-line long and straight ; cardinal extremities rounded. A distinct false 

 area is formed by a pair of ridges diverging from the beak toward the cardinal 

 extremities. On the interior the teeth are moderately well developed and 

 unsupported by dental lamellae. 



The brachial valve is depressed convex in the umbonal region and bears a 

 more or less conspicuous median sinus. The hinge-plate consists of two broad, 

 stout processes, diverging outwardly, grooved on their summits, and separated 

 from each other by a narrow, sharp cleft. They form both the socket walls 

 and crural bases, and are supported by a low median ridge. Muscular impres- 

 sions obscure in the typical species. 



