194 PALAEONTOLOGY OF NEW YORK. 



impunctate species, O. subquadrata, aiid the punctate species, 0. suhaquata, 0. 

 Michelini, 0. resupinata, and their allies. 



The Orthis 1 la,urentina, Billings, from the Anticosti group, is an interesting 

 species, similar in surface, contour and interior to Orthis calligramma. But in the 

 pedicle-valve the delthyrium is persistently covered with a convex plate without 

 trace of an opening, and in the brachial valve the deltidium is more or less 

 complete. 



PLECTORTHIS 



(nom. propos). 



II. Group of Orthis plicatella, Hall. 



PLATE V, FIGS.18-26. 



1847. Ortlds, Hall. Paleontology N. Y., vol. i, pp. 120-122, pi. xxxii, figs. 6, 7, 9. 



1861. OvtMa, MoChbsney. Desei-iption New Palaeozoic Fossils.Jp. 77. 



1861, OrtUs. Hall. Thirteenth Rept. N. Y. State Cab. Nat. Hist., p. 121. 



1861. Orthis, Hall. Fourteenth Rept. N. Y. State Cab. Nat. Hist., p. 90. 



1862. Orthis, cf. Treimtospira, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., pi. ii, tigs. 6-S. 



1863. Orthis, Billings. Geology of Canada, p. 165, fig. 145. 



1868. OrthAs, McChesnby. Transactions Chicago Acad. Sci., vol. i, p. 29, pi. ix, fig. 3. 



1872. Orthis, Meek. American Journal Science, vol. iv, p. 281. 



1872. Orthis, Meek. Paljpontology of Ohio, vol. i, pp. 106-109, pi. viii, figs. 6-S. 



1 1873. Orthis, Meek. Paleontology of Ohio, vol. i, p. 105, jil. viii, fig. 9. 



? 1874. Orthis, Billings. Palseozoic Fossils, vol. ii, p. 34, pi. 3, fig. 3 



1875. Orthis, Hall and Whitfield. Paleontology of Ohio, vol. ii {? p. 76, pi. i, fig. 20); p. 77, pi. i, 



figs. 21, 22. 



1875. Orthis, Miller. Cincinnati Quart. Jour. Sci., vol. ii, pp. 30-33. 



1879. Ort?iis, Ulrich. Journal Cincinnati Soc. Nat. Hist., vol. ii, p. 15, pi. vii, fig. 11. 



1881. Orthis, N. H. Winohell. Ninth Ann. Rept. Geol. and Nat. Hist. Surv. Minnesota, p. 115. 



1884. Orthis, Walcott. Palaeontology Eureka Dist., p. 73, pi. xi, fig. 5. 



This is a persistent form, which in American faunas, so far as known, is lim- 

 ited to the Trenton and Hudson River formations. While it retains the strong 

 external ribs of the typical Orthis, these are not invariably simple {0. fissicosta, 

 Hall ; 0. triplicatella, Meek ; 0. czquivalvis, Hall, not Davidson ; 0. Jamesi, Hall) ; 

 the cardinal area of the pedicle-valve is comparatively low and the valves are 

 subequally convex. In the interior the character of the muscular scars, dental 

 lamellae and cardinal process is essentially the same as in Group I, and the 

 minute structure of the shell appears to be in precise agreement with that of 

 0. calligramma, though no evidence of tubulose costa3 has been observed. In 



