NAPLES FAUNA IN WESTERN NEW YORK, PART 2 249 



Pterochaenia fragilis Hall (sp.) 



Plate 5, fig. 1-10 



Avicula fragilis Hall, Geology of New York; report on the fourth district 



1843. p. 222, fig. 94-1, 2 

 Lunulicardium fragile Hall, Preliminary Notice Lamellibranchiata. pt 2. 



1870. p. 97 

 Aviculopecten fragilis S. A. Miller, Catalogue North American Paleozoic 



Fossils. 1877. p. 184 

 Lunulicardium fragile Hall, Paleontology of New York. 1885. v. 5, pt 1, 



P- 434, pl- 7°, fig- !- T 4 

 Lunulicardium fragile Clarke, U. S. Geol. Sur. Bui. 16. 1885. p. 62 

 Lunulicardium fragile H. S. Williams, U. S. Geol. Sur. Bui. 41. 1887. p. 38, 

 Pl- 3, fig- 7 

 This species, which has already been fully described and illustrated by 

 Professor Hall (1885), has an unusual vertical range without wide variation. 

 The bituminous shales of the Marcellus division and those of the Genesee 

 division locally abound in specimens of like character. In the Hamilton 

 group the species occurs occasionally where the sediment becomes dark, 

 and in the shales of the Naples division it is among the less common lamel- 

 libranchs. A specimen in the State Museum collections shows its continued 

 presence after the introduction of the Chemung fauna. Among all its 

 appearances throughout its vertical range and its varying aspects, differences 

 in size are not often accompanied by material or persistent differences in 

 outline. It is undoubtedly to the fragility and tenuity of the shell that 

 some of the apparent differences are to be ascribed. Nevertheless, we 

 observe throughout the history of these shells that the extremes of varia- 

 tion in outline afford (1) the broadly spatulate or elongate form which is 

 the normal and most usual expression of the species, ana (2) a much more 

 orbicular or subquadrate shell. The history and relations of these extremes 

 are somewhat as follows. With the first appearance of the species in the 

 Marcellus shales the former of these prevails, and the second, or other pas- 

 sage forms between the two extremes are seldom met. The normal is well 



