230 
PALAEONTOLOGY OF NEW YORK. 
Genus BILL1NGSELLA, gen. nov. 
PLATE VHa, FIGS. 1-9. 
1857. Orthis, Billings. Report Geological Survey of Canada, p. 297. 
1S61. Orthisina, Billings. Geology of Vermont, vol. ii, p. 949, figs. 350-352. 
1861. Orthisina, Billings. Palaeozoic Fossils, vol. i, p. 10, figs. 11, 12. 
1862. Orthisina, Billings. Palaeozoic Fossils, vol. i, p. 138, fig. 115. 
1863. Orthis, Hall. Sixteenth Rept. N. Y. State Cab. Nat. Hist., p. 134, pi. vi, figs. 23-27. 
1863. Orthisina, Billings. Geology of Canada, p. 284, fig. 289. 
1867. Orthisina, Hall. Trans. Albany Institute, vol. v, p. 113. 
1882. 0>'this, Whitfield. Geology of Wisconsin, vol. iv, p. 170, pi. i, figs. 4, 5. 
1883. Orthis {Orthisina?), Hall. Report of State Geologist N. Y., pi. xxxvii, figs. 16-19. 
1884. Orthisina, Whitfield. Bulletin Am. Museum Nat. Hist., vol. i, p. 144, pi. xiv. fig. 6. 
1886. Orthisina, Walcott. Bulletin No. 30, U. S, Geol. Survey, pp. 120-122, pi. vii, figs. 5-7. 
This term is proposed to include certain species, few in number, according 
to present knowledge, which have some decided external resemblance to Orthis 
and Clitambonites (=Orthisina), but which must be excluded from these genera 
on the basis of internal characters. The form taken as typical of this genus is 
Orthis Pepina, Hall, of the Potsdam sandstone of Wisconsin ; other examples are 
Orthisina festinata, Billings, O. orientalis, Whitfield, and O. transversa, Walcott. 
Diagnosis. Shell transverse; subquadrate or semicircular in outline. Con¬ 
tour concavo- or plano-convex. Surface sharply striate or plicate. Pedicle- 
valve the more convex; cardinal area moderately high, vertical or slightly 
incurved. Delthyrium covered by a convex plate, which, in rare instances, 
may be minutely perforated at the apex; the teeth are well developed, but 
the dental plates are continued along the bottom of the umbonal cavity, 
enclosing a small subelliptical muscular area near the apex. In the brachial 
valve the cardinal area is greatly inclined, making an obtuse angle with that 
of the opposite valve. The delthyrium is partially covered by a convex 
deltidium which never attains the development seen in the opposite valve, and 
is often wholly absent. 
Type, Orthis Pepina, Hall. Potsdam sandstone. 
Observations. From the foregoing description it is evident that the 
characters of this group are essentially orthoid. The shells are, however, all 
small and bear in common an expression unlike that of Orthis, while the 
presence of a highly developed arched deltidium is a feature showing positive 
