80 PALAEONTOLOGY OF NEW YORK. 



Genus (or Subgenus) PENTAGONIA, Cozzens. 1846. 



PLATE XLIl. 



1841. Atrypa, Conrad. Fifth Ann. Rejit. Geol. Survey of N. Y., p. 56. 



1846. Pentagonia, Cozzen.s. Annala of the Lyceum of Nat. Hist, of N. Y., vol. iv, p. 1!)8, pi. x,fig. 3. 



1857. Rhynchonella, H.all. Tenth Kept. N. Y. State Cab. IVat. Hist., p. 125. 



1860. Athyris f, Billings. Canadian Journal, vol. v, p. 279, tig's. 39-42. 



1861. GoHiocoelia, Hall. Fourteenth Rept. N. Y. State Cab. Nat. Hist., p. 101. 



1862. Meristella, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., pi. ii, tig-s. 17-25 ; pi. xi, fig. 10. 



1863. Athyrix, Billings. Geology of Canada, p. 373, fig. 396. 



1864. Pentagonia, Meek and Hatdbn. Palaeontolog-y of the Upper Missouri, p. Hi. 



1867. MerUtella (Pentagonia), Hall. Palaeontology of N. Y., vol. iv, pp. 309-311, pi. 1, figs. 18-35. 

 1889. Meristella, Ne-itelroth. Kentucky Fossil Shells, p. 99, pi. xv, tigs. 9-16. 



Among the species of raeristelloid structure, an interesting variation in the 

 form of the shell is presented by the little M. lenta, Hall, of the Oriskany sand- 

 stone. Here the pedicle-valve is very shallow and so deeply sinuate as often 

 to appear concave ; the brachial valve is quite convex, especially along the 

 median line. A more extreme development of this form is seen in the Atrypa 

 unisulcata, Conrad, of the Corniferous limestone and the Hamilton group, and 

 it was to this species that the name Pentagonia was early applied by Isaachar 

 Cozzens.* The description of the genus was brief, but the accompanying 

 figures clear and unmistakable. This species is characterized by a very broad 

 sinus on the pedicle-valve, which is limited by divergent carinse, outside of 

 which the cardinal or lateral slopes are very abrupt. On the brachial valve is 

 a rounded median fold, which may be divided on its summit by a narrow sinus 

 and in the umbo-lateral region are two short folds or flanges, beginning at the 

 hinge-line, having a slightly sinuous curve and terminating before traversing 

 much more than one-third the length of the valve; occasionally there is a 

 second of these ridges on each side. 



The muscular impres.sions of the valves are essentially as in Meristella. 

 The hinge-plate has a peculiar structure ; it arises vertically from the bottom 

 of the valve, presenting an erect, concave anterior face, which is traversed by 

 a laint median ridge continuous with the septum of the valve. The posterior 



* Description of Three New Fossils from the Falls of the Ohio ; Annals of the New York Lyceum of 

 Natural History, vol. iv (1846), p. 158, pi. x, figs. 3«, b (Pmtagmiia Peevsi = Atrypa unisulcata, Conrad). 



