05 TRANS. OF THE ACAD. OF SCIENCE. 



This symmetrical shell is most like the /Sp. Plattensis, but 

 it is much smaller, wider and shorter in proportion, and more 

 angular in front. In all these particulars it differs from the 

 Sj). subtilita. 



This fossil is abundant in the Lower Coal Measures in 

 Boone and Audrain counties, where it was discovered by M. 

 G. Singleton, Esq. 



SriRIGERA TECTINIFERA. 



Shell very large, transverse, elliptical, widest in the mid- 

 dle, sinuate, lamellose. Ventral valve depressed, most con- 

 vex near the beak and on each side of the sinus in front, flat- 

 tened towards the lateral margins; beak large, prominent, 

 moderately incurved; foramen circular, oblique to the plane 

 of the margins of the valves; sinus deep, concave in front, 

 obsolete towards the beak. Dorsal valve more convex; 

 umbo large; beak incurved beneath the foramen; mesial /old 

 very prominent in front. /Surface ornamented with numer- 

 ous, thin, pectinated concentric lamella?, and distinct lines of 

 growth. 



Length, 2.51 ; breadth, 3.28. 



In general appearance this fossil resembles the Athyris 

 incrassata of Prof. Hall; but its markings clearly distin- 

 guish it. 



This rare shell I collected from the Geode Beds of the Ar- 

 chimedes Limestone at Keokuk, Iowa. 



SriRIGERA REFLEXA. 



Shell small, subrhombic, lamellose, widest above the mid- 

 dle, front subpointed or more sharply rounded. Ventral valve 

 depressed towards the margins, convex in the middle and 

 towards the beak, lateral margins turned down ; beak small, 

 moderately incurved ; foramen circular, small, plane oblique 

 to the margins of the valves. Dorsal valve more convex, 

 more regularly arched, fullest above the middle, slightly flat- 

 tened towards the lateral margins; umbo large and full; beak 

 rather small, strongly incurved beneath the foramen. Sur- 

 face ornamented by numerous, wide, concentric, pectinated 

 lamelhe ; those on the ventral valve less depressed than those 

 on the opposite valve. 



Length, .<>-!; breadth, .04; thickness, .86. 



This shell is most nearly allied to the 8pirigera America- 

 na ; but its depressed ventral valve, very convex dorsal 

 valve, and subrhombic form, give it permanent and import- 

 ant distinctions. It differs from the a. hirsuta in the same 

 particulars, and in having a shorter beak, and in being wider 

 above the middle ; it is also larger. Some might be disposed 



