SWALLOW DESCRIPTION OF NEW FOSSILS. 87 



width of the shell ; foramen small, triangular, higher than 

 wide; sinus obsolete or linear near the beak, but well devel- 

 oped in front. Dorsal valve less convex and nearly straight 

 alone; the middle from the umbo to the anterior margin, trans- 

 versely elliptical, flattened towards the lateral margins; umbo 

 full; beak small, scarcely projecting beyond the cardinal line. 

 Surface beautifully marked by line concentric and radiating 

 strife; the exfoliated shell and the cast show large unequal 

 radiating costa3. 



Length, .57 ; width, .65 ; thickness, .37. 



This beautiful fossil is rare in the upper divisions of the 

 Archimedes Limestone in Ste. Genevieve county, Missouri. 



Spirigera Plattensis. 



Shell of medium size, very gibbous, long ovate, subtrilo- 

 bate, junction of valves sinuate. Ventral valve very convex, 

 strongly arched; sinus large, prolonged in front; wnbo large; 

 beak short, strongly incurved; foramen circular. Dorsal 

 rah; most convex near the beak, high and rounded from the 

 umbo to the anterior margin; beak strongly incurved, cut- 

 ting the periphery of the foramen. Whole surface punctate, 

 ami marked with numerous, unequal, distinct lines of growth. 



Young shells less convex, ovate; margins nearly orbicular 

 and scarcely sinuate ; both valves convex near the beaks and 

 flattened towards the margins. 



Adult specimen— length, .88; width, .70; thickness, .55. 



Young specimen— length, .51 ; width, .47 ; thickness, .38. 



This shell is most like the 8. Hawni, but it has no longitu- 

 dinal costse or striae ; its lines of growth are more numerous 

 and more conspicuous. It differs from the S.subtilita in the 

 same particulars; it is also proportionally longer and thicker, 

 and the umboncs are larger. 



This beautiful shell is "common in the Upper Coal Meas- 

 ures of the north-western counties of Missouri, and in Kansas 

 and Nebraska. 



Spirigera Singletonii. 



Shell small, gibbous, broadly ovate, sinuate, subpolygonal 

 in front. Ventral valve ovate, very convex, strongly arched ; 

 umbo full ; beak large, strongly incurved; foramen circular; 

 sinus wide and deep in front, obsolete towards the beak. 

 Dorsal valve suborbicular, less convex, highest near the beak, 

 raised in front, depressed on each side of the mesial fold into 

 a rounded sinus ; umbo full', beak small, cutting the periph- 

 ery of the foramen. Whole surface marked with very dis- 

 tinct lines of growth. Structure punctate or fibrous. 



Length, .78; width, .72; thickness, .46. 



