SWALLOW DESCRIPTION OF NEW FOSSILS. 83 



TEREBRATULA GliACILIS. 



Shell ovate-lanceolate, widest near the anterior extremity, 

 punctate. Ventral valve strongly arched, flattened in front; 

 umbo convex; sides, towards the beaks, descending rapidly, 

 and drawn in to the junction with the other valve; beak long, 

 strongly curved in ; foramen elliptical ; sinus obsolete in 

 young shells, but more conspicuous in adults. Dorsal valve 

 very convex along the middle, and but little arched from the 

 umbo forward ; beak small and strongly incurved. Surface 

 very distinctly punctate ; punctures so arranged as to give 

 the appearance of oblique striations. 



Length, .87; width, .00; thickness, .40. 



This shell is very like the T. millepunctata, but it is less 

 arched, not so flat and wide in front, and the sinus of the ven- 

 tral valve is much less conspicuous. 



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

 edes Limestone at St. Mary's, Missouri, and Chester, Illinois. 



Terebratula parva. 



Shell small or very small, ovate, gibbous ; often very much 

 thickened on the margin, making the thickness greater than 

 the width. Ventral valve strongly arched, most convex near 

 the beak, slightly flattened towards the anterior margin, 

 thickened and contracted on the cardinal margin, rounded in 

 front; beak large, prominent, strongly incurved; foramen 

 small, circular. Dorsal valve shorter, more equally convex, 

 thickened on the anterior margin ; umbo large; beak strongly 

 incurved. Surface marked with fine radiating and concen- 

 tric striae, and a few indistinct lines of growth, which are 

 most numerous and distinct on the margin. 



This shell is very variable in size and form, as the follow- 

 ing measurements show : 



Shell of ordinary form— length, .87; width,. 27; thickness, .26. 



Shell of gibbous form — length, .50; width, .28; thickness, .40. 



Shell of wide form — length, .43 ; width, .35 ; thickness, .28. 



This fossil is found at Keokuk, Iowa, and in Monroe and 

 Cooper counties, Missouri. 



Terebratula arcuata. 



Shell long, lanceolate, arcuate, sinuate, punctate, broadest 

 below the middle. Ventral valve strongly arched, flattened 

 on top, contracted and thickened on the hinge line and late- 

 ral margins; sinus broad and shallow in front; beak long and 

 strongly incurved ; foramen elliptical. Dorsal valve very 

 convex, nearly straight from the umbo two thirds of the dis- 

 tance to the anterior margin, then curved abruptly down to 



