194 
PALEONTOLOGY OF NEW YORK. 
impunctate species, 0. subquadrata, and the punctate species, 0. subccquata, 0. 
Michelini, 0. resupinata, and their allies. 
The Orthis ? laurentina, 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. Orthis, Hall. ' Palaeontology N. Y., vol. 5, pp. 120-122, pi. xxxii, figs. 6, 7, 9. 
1861. Orthis, McChesney. Description New Palaeozoic Fossils,"p. 77. 
1861, Orthis, 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. Trematospira, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., pi. ii, figs. 6-8. 
1863. Orthis, Billings. Geology of Canada, p. 165, fig. 145. 
1868. Orthis, McChesney. Transactions Chicago Acad. Sci., vol. i, p. 29, pi. ix, fig. 3. 
1872. Orthis, Meek. American Journal Science, vol. iv, p. 281. 
1872. Orttiis, Meek. Palaeontology of Ohio, vol. i, pp. 106-109, pi. viii, figs. 6-8. 
1 1873. Orthis, Meek. Palaeontology of Ohio, vol. i, p. 105, pi. viii, fig. 9. 
1 1874. Orthis, Billings. Palaeozoic Fossils, vol. ii, p. 34, pi. 3, fig. 3 
1875. Orthis, Hall and Whitfield. Palaeontology of Ohio, vol. ii (1 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. Orthis, Ulrich. Journal Cincinnati Soc. Nat. Hist., vol. ii, p. 15, pi. vii, fig. 11. 
1881. Orthis, N. H. Winchell. 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 (O. fissicosta, 
Hall; O. triplicatella, Meek; O. aquivalvis , Hall, not Davidson; O. 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 
O. calligramma, though no evidence of tubulose costae has been observed. In 
