598 GEOLOGICAL SURVEY OF IOWA. 



Chonetes logani. 



PLATE XII. FIG. 1 a - e, and 2. 



Chonetes logani : NORWOOD & PRATTEK, Journal of the Academy of Natural Sciences 



of Philadelphia, 1854, Vol. iii, p. 30. 



SHELL small, semielliptical : hinge-line scarcely equalling 

 the width of the shell below. Ventral valve convex, gibbous 

 in the middle and on the umbo ; the cardinal margin fur- 

 nished with five or six oblique spines on each side of the 

 beak. Dorsal valve slightly concave, the concavity much less 

 than the convexity of the opposite valve. 



SURFACE marked by fine rounded dichotomizing strise, of 

 which from one hundred to one hundred and twenty or more 

 may be counted on each valve, dependent upon the size of 

 the shell. Vascular impressions in the dorsal valve large, 

 doubly ovate, and separated by a longitudinal ridge which 

 reaches from the beak half way to the base. 



This species bears some resemblance to C. variolata ; but it is proportionally 

 longer, while the hinge-line is less extended, and there is no appearance of a sinus in 

 the middle of the ventral valve. It approaches in form very nearly to C. shumardiana ; 

 but is more coarsely striated, and does not possess the fine granulations so charac- 

 terestic of that species. 



Fig. 1 a. Ventral valve of a small specimen. 



Fig. 1 b. Profile of the same. 



Fig. 1 c. Dorsal valve of another individual. 



Fig. 1 d. Enlargement of a ventral valve, showing the character and number of the striae. 



Fig. I e. A farther enlargement of the surface, showing the radiating and fine concentric 

 striae. 



Fig. 2. Interior of a small dorsal valve, showing the vascular impressions. 



Geological formation and localities. In the Burlington limestone : Bur- 

 lington, Iowa ; Quincy, Illinois. 



Productus flemingi, var. burHngtoneiisis. 



PLATE XII. FIG. 3 a - g. 



Productus flemingi : SOWERBY, Min. Conchology, 1812, Vol. i, pa. 155, pi. 68, f. 2. 

 P .flemingi : DE KONINCK, Monograph of the Genus Productus, etc. pa. 95, pi. x, f. 2. 

 For synonymy and references, see DE KONINCK as above. 



SHELL of medium size or larger, longer than wide, some- 

 times length and breadth equal : hinge-line less than the 



