230 PALEONTOLOGY OF NEW YORK. 



Genus BILLINGSELLA, gen. nov. 



PLATE VUa, figs. 1-9. 



1857. Orthis, Billikgs. Report Geological Survey of Canada, p. 297. 



1861. OrtMsina, Billings. Geology of Vermont, vol. ii, p. 949, figs. 350-352. 



1861. OrtTiisina, 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 Kept. 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. Ti-ans. Albany Institute, vol. v, p. 113. 



1882. Orthis, 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, 0. orientalis, Whitfield, and 0. 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 



