658 GEOLOGICAL SURVEY OF IOWA. 



Rhynchonclla mutata. 



PLATE XXIII. Fio. 2 a, b. 

 Rhynchonella mutata : HALL, Transactions of the Albany Institute, Vol. iv, 1856. 



SHELL subtrigonal, more or less gibbous; front broadly 

 rounded or nearly straight, abruptly tapering to the apex, 

 the two sides meeting at an angle of nearly 90. Dorsal valve 

 much more convex than the opposite one, which is often 

 depressed ; shell most convex near the anterior margin ; beak 

 obtusely angular, and closely incurved. Beak of ventral valve 

 nearly straight, or but slightly incurved : sinus not deeply 

 impressed on the margin of the shell ; foramen triangular. 



SURFACE marked -by from twelve to sixteen strong sub- 

 angular plications, about four or five of which are depressed 

 in the sinus of the dorsal valve : concentric striae rarely 

 visible. 



Length, -15 to -30, width -14 to -82 of an inch. 



Fig. 2 a, b. Dorsal and ventral views of a specimen of ordinary size. 

 Geological formation and locality. In the Warsaw limestone near Alton, 

 Illinois. 



Rhynchonella subcuneata. 



PLATE XXIII. FIG. 3 a, b, c. 

 Rhynchonella subcuneata : HALL, Transactions of the Albany Institute, Vol. iv, 1856. 



SHELL triangular, subcuneate ; front rounded, meeting the 

 lateral slopes at an obtuse angle ; sides sloping to the beak, 

 and meeting at an angle of 60 or 65 : valves nearly equally 

 convex. Ventral valve most convex towards the beak which 

 is very acute, scarcely incurved, and perforated by a triangular 

 foramen : sinus obscure : beak of dorsal valve acute, closely 

 incurved below the triangular foramen. 



SURFACE marked by about twelve to fourteen (and rarely 

 sixteen) strong simple angular plications, which are some- 

 what obsolete near the beak. Plications crossed by fine con- 

 centric striae, and, in old shells, at irregular distances, by 



