300 PALAEONTOLOGY OF NEW YORK. 



There are at least three well known species which are clearly referable to 

 this proposed genus : Leptana subquadrata. Hall,* from Perry county, Tennessee, 

 referred to the age of the Lower Helderberg group ; Leptana tenuicincla, McCoy, 

 elaborately described by Mr. Davidson,! from the Upper Llandeilo and the 

 Caradoc series ; Pledambonites ovata and P. oblonga, Pander, which are united 

 by LIE V'ERNEUiLt uuder the terra Leptana oblonga, from the Lower Silurian 

 beds in the vicinity of St. Petersburg. Davidson includes in L. tenuicinda the 

 Lepfana ccnigma of de Verneuil,§ from the Lower Silurian beds of the Island of 

 Gotland, a shell much less elongated than the typical L. tenuidnda, and whose 

 interior has not been described. 



Genus LEPT^NISCA, Beecher. 1890. 



PLATE XV, FIGS. 30,31; AND PLATE XVA, FIGS. 19-30. 



1859. Le.pt(Ena, Hall. Paleontology of N. Y., vol. iii, p. 197, pi. xviii, fig. 2, 



1SS3. Leptoma .» (sub-genus 1), Hall. Kept. N. Y. State Geologist for 1882 ; Expl. pi. xv (46), tigs. 30, 31. 



1890. Leptmnisca, Beecher. American Journal of Science, vol. xl, p. 238, pi. ix, figs. 1-9. 



Diagnosis. "Shell concavo-convex, attached to foreign objects by calcareous 

 cementation of the ventral beak. Valves articulated by the teeth and sockets. 

 Dorsal or .socket valve concave ; interior with a broad, more or less defined, 

 spiral impression on each side of the median line, making a single volution. 

 Adductor impressions small. Cardinal line [area] narrow, bearing in the center 

 two prominent, bilobed, cardinal processes, separated to admit the vertical 

 septum in the opposite beak. Ventral valve convex, area elongate-triangular, 

 fissure covered with a pedicle-sheath. Cardinal muscular scar supported on, or 

 limited by a vertical septum, on each side of which, in the anterior half, is a 

 small adductor scar. Shell-structure punctate. 



" Type, Leplmnu concava, Hall."* Lower Helderberg group. 



This interesting genus includes a few species of attached shells having the 

 general form and contour of Plectambonites, but with a peculiar combination 



* This species is first mentioned and illustrated in the Report of the N. Y. State Geologist for 1882, 

 explanation of plate xv (46), figs. 32, 33. 1883. 



t British Silurian Brachiopoda, pji. 326-328, pi. xlvii, figs. 7-18. 



J Geologie de la Russie, etc., p. 228, pi. xv, fig. 2. 



§ Bulletin de la Soci6t6 Geologique de France, 2e Ser., vol. v, p. 840, pi. iv, tig. 6. 1848, 



I Bbbchkk. "On Lept^nisoa, a newjgenus of Brachiopod from the Lower Helderberg group " (op. cit.) 



