632 Systematic Paleoxtology — UrrER Devonian 



exfoliation frequently shows evidence of fine radial lines on the inner 

 shell layer. 



The type is Avimla fragilis Hall. 



Pteroch^xia fkagilis (Hall) 

 Plate LXII, Figs. 5-7 



Avicula fragilis Hall, 1843, Geol. of N. Y., Kept. 4th Dist., p. 222, lig. 94 



(1, 2). 

 Lunulicardium fragile Hall, 1870, Preliminary Notice Lamellibr., pt. ii, p. 97. 

 Lunulicardium fragile Hall, 1885, Pal. of N. Y., vol. v, pt. i, p. 434, pi. Ixxi, 



figs. 1-14. 

 Lunulicardium fragile Clarke, 1885, Bull. U. S. Geol. Survey, No. 16, p. 62. 

 Lunulicardium fragile Williams, 1887, Bull. U. S. Geol. Survey, No. 4, p. 38, 



pi. iii, fig. 7. 

 PterocTiwnia fragilis Clarke, 1904, N. Y. State Mus., Mem. vi, p. 249, pi. v, 



figs. 1-10. 



Description. — Shell small, somewhat obovate, often subcircnlar. Ob- 

 liquely truncate in front, the margins gaping and bounded by thin 

 expansions. Valves moderately convex with rather abrupt posterior slope. 

 Beaks attenuate, erect or progyre. Surface with fine concentric striae, 

 radial stride only in internal laminge. 



This peculiar and widely distributed species, the only described repre- 

 sentative of its generic type of structure has already been pretty fully 

 made known in the works above cited. It has an unusual vertical range 

 without wide variation of the specific type, occurring first in the bitum- 

 inous shales of the Marcellus stage, occasionally in the more calcareous 

 beds of the Hamilton and again in the dark shales of the Genesee. In the 

 Styliola limestone of the latter divisions it is profusely abundant wliile 

 in the argillaceous beds carrying the Naples fauna it is one of the less 

 common species of pelecypods. It is known also to have entered the 

 Chemung fauna of western New York, a survivor of the migration of the 

 Naples fauna from that region. Notwithstanding its extensive vertical 

 range in the Devonian strata of western New York the species is not 

 abundant west of the Naples subprovince of Portage time. In Maryland 

 the species seems to be quite widely dili'used. 



Length 6 mm. ; height 7 mm. 



