154 PALEONTOLOGY OF NEW YORK. 



Genus ZYGOSPIRA, Hall. 1862. 



PLATE LV. 



1847. Atrypa. OrtUs ?, Slmocisma, Hall. Paleontology of N.Y., vol. i, vp. 140-142, 288, pi. xv, fig. 15 ; 

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



1859. RhyncJionellah Hall. Twelfth Kept. N. Y. State Cab. Nat. Hist., p. 66. 



1860. Atrypa, Hall. Thirteenth Kept. N. Y. State Cab. Nat. Hist., p. 69. 



1862. Zygospira, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., p. 154, figs. 1, 2. 



1862. Zygospira, Billings. Canadian Naturalist and Geologist, vol. vii, p. 393. 



1862. Athyris, Billings. Palieozoic Fossils, vol. i, p. 147, tigs. 125-127. 



1S63. Athyris, RhyncfKmella ?, Billings. Geology of Canada, p. 168, fig. I.'>2; p. 211, fig. 211 ; p. 212, 



fig.s. 214-216. 



1864. Sfenocixma, Meek and Haydbn. Paleontology of the Upper Missouri, p. 16. 



1866. RhynchoneUa, Zygospira, Billings. Catalogue Silurian Fossils of Anticosti, pp. 44, 46. 



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



1868. Zygospira, Mekk. 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, Meek. Paleontology of Ohio, vol. i, pp. 125, 126, pi. xi, figs. 4, 5. 

 1875. Zygospira. Millkk. Cincinnati Quarterly Journal of Science, vol. ii, pp. 58, 59. 



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



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

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



1882. Zygospira, Anazyga, Davidson. British Silurian Brachiopoda, Supplement, pp. 122, 128. 



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



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

 1893. Zygospira, Winchell and Schcchekt. 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. 



