No. 109. 



153 



Spirigera spiriferoides. 



Terebratula spiriferoides, Eaton : Silliman's Journal, 1831. Vol. x.\i, p. 137; 



Geol. Text-book, p. 4G, 1832. 

 Atrypa concentrica, Coxrad (not T . concentrica, Bronx) : Aun.Rcp.N.Y. 



18 J«, p. m,ii-j. 

 A. concentrica : Final Rep. 4th Dist. N.Y., p. 198, f. 5. 



Shell varying from transversely oval to suborbicnlar ; young 

 individuals rather compressed, adult specimens gibbous; hinge 

 nearly straiglit, but rounded at the extremities : dorsal valve 

 generally a little more convex than the other, most prominent 

 near the middle, elevated in front into a broad rounded unde- 

 fined mesial fold, Avhich is usually indistinct, but sometimes 

 strongly elevated near the border ; beak and umbonal region 

 not extending much above the hinge line : ventral valve most 

 convex near tlie beak, depressed in front into a shaHow rounded 

 sinus, which is sometimes faintly indicated nearly to the beak, 

 and occasionally very deep at the border; beak small, not pro- 

 jecting far beyond that of the other valve, lying close against 

 it but scarcely incurved, perforated by a small round aperture. 

 Surface marked by distinct concentric imbricating lamellae of 

 growth. 



This species has generally been referred to Terehrahda ( Spirigera ) 

 co7icentrica of Broxn, from which it differs in the straightness of its hinge 

 and much less prominent beak. It also presents other slight but constant 

 differences of form, the broadest part being almost always a little above 

 the middle, while in S. concentrica it is generally a little below it. The 

 lamellse are likewise more distinctly imbricated in our shell than in S. 

 conce7ilrica. A comparison of the European specimens with American ones 

 has shown the propriety of separating them, and of adopting the name given 

 by Prof. Eaton in 1831. 



Spirigera spiriferoides. 



