84 TRANS. OF THE ACAD. OF SCIENCE. 



the junction with the other valve; umbo large. Surface 

 smooth, closely punctate, with a few very indistinct lines of 

 growth. 



This shell is somewhat like the T. gracilis ; but it is 

 much larger, more arcuate, and the dorsal valve is much more 

 gibbous. 



The T. arcuata is common in the Kaskaskia Limestone in 

 Ste. Genevieve county, Missouri, and at Chester, Illinois, 

 where it is associated with the S. incrassatus. 



Terebratula brevilobata. 



Shell small, elongate, ovate, gibbous, punctate, and lobate. 

 Ventral valve very convex near the beak, strongly arched lon- 

 gitudinally ; beak long, strongly incurved ; foramen ovate or 

 elliptical; sinus well defined and deep on the anterior por- 

 tion, obsolete or wanting towards the beak. Dorsal valve 

 shorter, very convex; umbo large; beak incurved beneath 

 the foramen. This valve usually has a short mesial fold in 

 front, with a deep narrow sinus on each side of it, dividing 

 the front of the valve into three lobes, the middle being much 

 the smallest. Surface beautifully punctate and marked by 

 lines of growth. 



Length, .60; breadth, .46; thickness, .35. 



Variety a is smaller, wider, and thicker in proportion. 



This shell is common in the upper divisions of the Archim- 

 edes Limestone in Ste. Genevieve county, Missouri. 



ElIVNCOXELLA ARCTIROSTRATA. 



Shell triangular or cuneate, valves nearly equal, costate, 

 striate. Ventral valve most convex towards the beak, which 

 is long, pointed, and strongly incurved. Dorsal valve most 

 convex in front ; beak small, pointed, and strongly incurve 1. 

 Both valves flattened in the middle, and bent abruptly near 

 the margins, forming perpendicular subrectangular faces on 

 the sides and one more or less convex and rounded on the 

 front. Each valve marked with from 14 to 16 rounded, ra- 

 diating plications, which extend from the beak — two or three 

 implanted — and are ornamented by line longitudinal stria*, 

 and by obsolete concentric folds. The angle at the beak very 

 variable. 



Obtuse specimen — length, .65; breadth, .SO; thickness, .43. 



Acute specimen — length, .60; breadth, .61; thickness, 45. 



This shell is somewhat similar to the T.subcuneata ot Prof. 

 Hall, but there are several important differences. The arcti- 

 rostrata has the beak strongly incurved; the fiat surfaces on 

 the sides larger, more rectangular, wider in front, and marked 

 with the costse curved over them. 



