ORISKANY SANDSTONE. 485 



Spirifer arenosus. 



Plate XCVIII. Ficr. 1 - 8 ; Plate XCIX. Fig. 1 - 10 ; and Plate C, Fig. 1-8. 



DellhyrU arenosa : Conkad, Annual Report on the Palasontology of New-York, ]839, p. 65. 



D. . — Vaxtjxem, Report on the Third Geological District, 1843, p. 123, f. \. 



D. — JLvTHEa, Report on the First Geological District, 1843, p. 342, f. 1. 



D — Hall, Report on the Fourth Geological District, 1843, p. 148, f. 1. 



Shell, in the young state, semielliptical, with the beak a little elevated 

 above the hinge-line ; the old shell becoming ventricose, and the beak 

 much elevated above the hinge-line, having a somewhat semioval form. 

 The proportions vary from length and width equal, to width one-third 

 greater than the length. Cardinal angles sometimes produced in acute 

 terminations, but usually rounded, particularly in old shells. 



Ventral valve very regularly convex, the greatest convexity about one- 

 third the distance below the beak. The cardinal angles, when produced, 

 are a little flattened. The mesial sinus is very shallow, being often little 

 more than a flattening of the surface along that part of the shell, The 

 umbo is broad, and the beak a little incurved over the area. Area in 

 young shells narrow, and in old shells proportionally wider, and ex- 

 tending to the cardinal extremities : the foramen is wide, and, in old 

 shells, partially closed above. The dental lamellae are strong; the 

 extremities, rising above the cardinal line and bending backwards 

 beneath the area, are widely divergent as they extend from the beak 

 downwards. The muscular impressions are large, and very strongly 

 and beautifully marked. 



Dorsal valve of the same form as the ventral ; its greatest convexity in 

 the middle, with a very narrow cardinal area, above which the beak is 

 slightly incurved. Mesial elevation very moderate, and sometimes 

 scarcely defined. 



Surface marked by regular simple rounded or sometimes subangular 

 plications, of which there are from ten to twenty on each side of the 

 mesial fold and sinus. The mesial sinus is simple at the apex ; but a 



[ PALiEONTOLOOY III.] 54 



