PALEONTOLOGY OF IOWA. 659 



stronger imbricating folds or wrinkles parallel to the lines 

 of growth. Sides of both valves, beneath the beak, free 

 from plications, and forming a very distinct elongate oval 

 space. 



Length, -16 to -41, width -15 to -39 of an inch. 



Fig. 3 a, b. Ventral and dorsal views of a specimen. 

 Fig. 3 c. Profile of the same. 



Geological formation and localities. In limestone of the age of the War- 

 saw limestone : Bloomington and Spergen hill, Indiana. 



Tereforatula trinuclea. 



PLATE XXIII. FIG. 4 a, b, c, and 5. 

 Terebratula trinuclea : HALL, Transactions of the Albany Institute, Vol. iv, 1856. 



SHELL subpentagonal or ovate, robust, trilobate, lobes 

 nearly equal : valves nearly equal ; the ventral one gibbous 

 towards the beak ; sinus beginning above the middle of the 

 valve, gradually becoming wider and deeper towards the 

 base ; in some specimens, distinctly bounded by an obtusely 

 angular ridge. Dorsal valve varying from subcircular to 

 transversely oval and longitudinally ovate, most convex be- 

 tween the centre and the beak, and distinctly trilobate ; 

 lobes extending about half way to the beak, the middle lobe 

 often marked by a distinct linear depression : beak of 

 ventral valve strong, rounded and incurved, truncated verti- 

 cally by a distinct rounded foramen. 



SURFACE marked by fine concentric lines, which undulate 

 with the lobes, and are extremely sinuous near the margin 

 of the shell. 



Old shells are often marked by strong imbricating lamellse 

 at unequal distances. 



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



This species bears a very close resemblance to Jlthyris (?) triloba of M'Cor 

 ( Synopsis of the Carboniferous limestone fossils of Ireland, pa. 149, pi. xx, f. 21), 

 particularly the old shells, which are broader and more strongly marked than the 

 others. 



