7 



face marked by fine concentric lines of growth, and some- 

 times by parallel stronger folds or wrinkles. Under the 

 magnifier the shell presents a finely punctate structure. 



Length, '14 to '44, width *10 to *31 of an inch. 



Locality. — Alton, 111. ; Bloomington, Spergen Hill, la. 



i> Terebratula trinuclea. — Shell sub-pentagonal or ovate, 

 robust; trilobate, lobes nearly equal; valves nearly equal, the 

 dorsal one gibbous towards the beak, a sinus in the centre, 

 beginning above the middle of the valve, gradually becom- 

 ing wider and deeper towards the base, in some specimens 

 distinctly bounded by an obtusely angular ridge. Ventral 

 valve varying from sub-circular to transversely oval and lon- 

 gitudinally ovate, most convex between the centre and the 

 beak, and distinctly trilobate, lobes extending about half way 

 to the beak ; the middle lobe often marked by a distinct li- 

 near depression ; beak of dorsal valve strong, rounded and 

 incurved, truncated vertically by a distinct rounded fora- 

 men. Surface marked by fine concentric lines, which undu- 

 late with the lobes, and are extremely sinuous near the mar- 

 gin of the shell. 



Old shells are often marked by strong imbricating lamel- 

 lae at unequal distances. 



Length, '20 to '51, width *19 to '46 of an inch. 



Locality. — Spergen Hill ; Bloomington ; la. 



SPIRIFERA, Sowerby. 

 Spirifera Xorwoodana. — Shell small, semi-elliptical, very 

 gibbous, angles rounded ; hinge line less than the greatest 

 width of the shell. Dorsal valve very convex and strongly 

 arching near the beak, which is curved over the area ; pli- 

 cations, about eight, the central ones very strong, and the 

 mesial depression distinctly continued to the beak. Ventral 

 valve ranging from depressed-convex to extremely convex, 

 and marked by three strong plications on each side of the 

 mesial fold, which has often a depressed line along the cen- 

 tre towards the base, with scarcely a distinct fold in the si- 

 nus of the dorsal valve. Area small, high, not extending 



