SWALLOW — DESCRIPTION OF NEW FOSSILS. 89 



to make this fossil and the S. Clintonensis mere varieties of 

 the S. Americana, but the differences are marked, important 

 and permanent; and, as it is more convenient "to know them 

 by name," they may as well stand among the species, at least, 

 until naturalists do better agree upon what shall constitute a 

 species. 



This fossil is rare at Barrett's Station, St. Louis county, 

 Missouri, where it is associated with many fossils of the 3d 

 Archimedes or Warsaw Limestone. 



Spirigera Clintonensis. 



Shell small, transverse, elliptical, depressed, subtrilobate, la- 

 mellose. Ventral valve most convex near the beak, flattened 

 towards the lateral margins ; sinus broad and deep on the an- 

 terior portion ; beak small, incurved, truncated nearly parallel 

 to the margins of the valves; foramen small, circular. Dorsal 

 valve flattened towards the lateral margins, most convex near 

 the middle ; mesial fold broad, full, and rounded from the 

 beak to the anterior margin, with a deep round sinus on each 

 side in front, giving the shell a trilobate form and sinuous 

 margin; the fold is often flattened or depressed along the 

 middle; beak small, curved beneath the foramen, cutting its 

 periphery. Spiral appendages large, 12 to 14 volutions on 

 each side. Surface ornamented with numerous concentric, 

 pectinated lamellse. 



Shell of common form— length, .60; width, .78; thickness, 

 36. Shell of the gibbous variety— length, .00 ; width, .70 ; 

 thickness, .41. 



This fossil is most nearly allied to the S. Americana ; but 

 it is much wider, more trilobate, and has more volutions in 

 the spiral appendages. 



This beautiful fossil is common at Chester, Illinois, and in 

 Ste. Genevieve and Cooper counties, Missouri, in the 2d Ar- 

 chimedes or Ste. Genevieve Limestone, associated with Spi- 

 rifer Zeidgi, S. spinosus, and Spirigera Americana. 



Spirigera Americana. 



Shell small, lenticular, wide, obovate or slightly transverse ; 

 widest at or above the middle, lamellose. Ventral valve con- 

 vex ; highest near the beak; flattened towards the margins; 

 sometimes slightly depressed, forming an indistinct sinus on the 

 anterior portion : beak small, moderately incurved; foramen 

 small, circular, oblique t<> the margins of the valves. Dorsal 

 valve transversely elliptical, more convex ; high and rounded 

 from the beak to the anterior margin, with, sometimes, a slight, 

 longitudinal mesial depression on the anterior portion; flat- 

 tened towards the lateral margins: beak small and curved 



