of Burlington, loiva. and its Vicinity. 227 



considerable distance from the margin, runs a slight depres- 

 *sion, giving them a flattened appearance. 



Fine concentric strias and imbricating lines of growth 

 are visible on well-preserved specimens. Where the shell 

 is partially exfoliated, it usually presents under the lens a 

 fine pustulose appearance. 



Length six lines, breadth seven lines. 



Geological formation and locality. In beds Nos. 1, 2, 3, 

 4, 5, 6, and the lower portion of No. 7, of the preceding 

 sections, Burlington, Iowa. 



XUCLEOSPIRA BARRISII (White.) 



Shell transversely oval, gibbous, becoming ventricose 

 with age. Hinge-line short, surface traversed by a few 

 imbricating lines of growth, which increase in number 

 near the border in the older specimens. Ventral valve 

 with a narrow, faintly impressed sinus extending from the 

 beak along the shell, corresponding to the inner septum, 

 which gradually expands into a broader and deeper depres- 

 sion, and, with a corresponding elevation in the opposite 

 valve at the margin, gives it considerable sinuosity in 

 front. Beak short, acute, and slightly incurved. A mi- 

 nute, round foramen just beneath the apex. 



False area small, concave. Longitudinal septum not 

 extending beneath the beak, but ending about even with 

 the cardinal teeth. Dorsal valve more gibbous than the 

 ventral, umbo prominent, longitudinal septum extending 

 the full length of the shell, but becoming indistinct at the 

 front margin. A narrow, scarcely perceptible impression 

 extends along the back, opposite the septum. The spatu- 

 late portion of the cardinal process short, and bending 

 slightly upward, to correspond to the under side of the con- 

 cave area, beneath which it passes at nearly a right angle 

 to the basal portion. The crura, being very small, serve to 



