512 GEOLOGICAL SURVEY OF IOWA. 



extremities, striated transversely ; foramen broadly triangu- 

 lar, the base greater than the height. Dorsal valve regularly 

 convex, flattened towards the hinge extremities ; mesial 

 elevation very moderate, marked by two plications above, 

 which dichotomize, becoming four or five below. 



SURFACE marked by from twelve to twenty rounded and 

 little elevated plications on each side the mesial fold and 

 sinus. Plications irregularly dichotomizing, sometimes near 

 the beak and sometimes near the base ; concentrically marked 

 by fine lamellose striae : the entire surface granulose, with 

 close slender radiating striae. 



This species is described and figured by Dr. SHUMARD in the Missouri Report for 

 1855. The fossil presents in different localities some varieties of surface markings, 

 according t<^the description cited above and a comparison of specimens in our collec- 

 tion. The narrow linear area, mucronate cardinal extremities, shallow sinus and 

 slight mesial elevation which are covered by dichotomizing plications, serve to di- 

 stinguish it from most other species. Spirifer parryanus, however, simulates this 

 one in its dichotomizing plications on the mesial fold and sinus ; while the plications 

 on the lateral portions of the shell are always simple, so far as observed. From this 

 similarity, I had at one time supposed the Spirifer marionensis to be found among 

 the Burlington fossils; but a later comparison has not shown any specimens. 



Fig. 1 a, b. Ventral and dorsal view of a specimen of medium size, in which one of the 

 cardinal extremities is produced into a mucronate point, the other being 

 being broken off. 



Fig. 1 c. Cardinal view of the same, showing the narrow linear area. 



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

 milton group : Louisiana and Hannibal, Missouri. 



Cyrtia umbonata ( n. s.). 



PLATE V. FIG. 2 a, b, c. 



SHELL somewhat semiglobose, with the hinge extremities 

 more or less salient. Ventral valve very gibbous in the 

 middle, arcuate ; beak incurved ; sinus broad, shallow above 

 and deeper below, limited on each side by a rounded or 

 obtusely subangular fold, and produced in front ; lateral 

 extremities auriculate : area broad, concave ; foramen nar- 

 row, closed by a pseudo-deltidium. Dorsal valve semiellip- 

 tical, more or less produced and flattened at the cardinal 



