NAPLES FAUNA IN WESTERN NEW YORK, PART 2 247 



Observations. This is a unique shell in this congeries. I have 

 referred it with some hesitation to the genus Opisthocoelus Beushausen, 1 

 which was erected for species having well defined vertical areas in front and 

 behind the triangular ligament area. 2 Shells of this genus are chiefly from 

 the lower Upper Devonic. 



Habitat. Genesee province ; Naples subprovince. In the Naples 

 shales at Ithaca N. Y. 



PTEKOCHAENIA g en - nOV. 



Among the shells which heretofore have been referred to the genus 

 Lunulicardium is a small species with smooth exterior occurring freely in 

 the Middle and Upper Devonic of New York and originally described by 

 Hall as an A vie u la fragilis. 3 Under this name have been covered all 

 the expressions of the species which are known to occur in the Marcellus, 

 Hamilton, Genesee and Portage shales. These shells have been abun- 

 dantly illustrated on plate 71 of Paleontology of New York, v. 5, pt 1, 

 and the figures there given (1-14) show excellently both the generic and 

 specific characters. The relation of these shells to Lunulicardium is not 

 remote, but there are palpable differences. The valves are, first, extremely 

 tenuous, being apparently little more than a film of lime which may 

 be considerably phosphatic. They bear no exterior ornamental mark- 

 ings save the fine concentric lines of growth ; all radial lines lie on the 

 inner surface of the valves and may be visible by translucence from without. 



The beaks are distinctly prosogyre ; on the anterior margins and just 

 in front of the beak is a conspicuous byssal hiatus which may extend for 

 one half the shell's length. This is bordered by two flanges, or narrow 

 explanate processes, which are widest at the beak, rapidly becoming 

 narrower toward the antelateral margin of the valves. Their surface is 

 either convex or their outer margins are decidedly elevated and they are 

 directed not inward nor vertically but outward in the plane of the valves. 



1 Op. cit. p. 338. 



2 See figures of the type species O. concentricus Beus., op. cit. pi. 38, fig. 9-1 1. 

 sGeol. N. Y. 4th dist. 1843 ; subsequently referred to Aviculopecten by S. A. Miller 



in Cat. Am. Paleozoic Foss. 1877. 



