PALAEONTOLOGY OF IOWA. 675 



SURFACE marked by round salient costse, which are di- 

 chotomous at intervals ; the point of bifurcation marked by 

 an elongated tubercle, from which originates a spine. This 

 character is more conspicuous on the middle and front of the 

 shell. On the upper part of the shell, the costse are crossed by 

 prominent concentric wrinkles, which are more conspicuous 

 on the auriculate cardinal extremities, where they are marked 

 by minute round tubercles or spine-bases : these give a some- 

 what nodose character to the costae over the upper portion 

 of the shell. Fine concentric striae mark the entire surface. 



The differences in character between this species and P. wortheni have already 

 been pointed out; and the two shells, though approaching each other in general form, 

 are conspicuously distinct. 



I am indebted to Dr. PROUT, of St. Louis, for the means of illustrating this species. 

 For a more full description and illustration of the same, see the paper cited at the 

 head of this description. 



Fig. 3 a. Ventral valve of a specimen of this species. [ The figure represents the shell 



too broad, and the sides too much curved.] 

 Fig. 3 b. Profile view of the same, showing the arcuate form and prominence of the 



marginal cincture. 

 Fig. 3 c. Dorsal valve of another individual. 



Geological position and localities. In the St. Louis limestone : St. Louis, 

 Missouri ; Milan, Illinois. 



Productus tenuicostus ( n. s.). 



PLATE XXIV. FIG. 2 a, b, c, d. 



SHELL somewhat ovoid, usually much longer than wide, 

 extremely arcuate above, the summit of the arch nearer the 

 beak. Dorsal valve convex near the beak and depressed on each 

 side, flattened in the middle and suddenly bent ipwur is m 

 front. Ventral valve extremely arcuate, rounded and gibbous 

 above, flattened or scarcely depressed and sometimes a little 

 elevated in the middle, spreading and auricalate at the cardi- 

 nal margins, extremely produced in front, expanded and 

 somewhat longitudinally plicate. 



SURFACE marked by numerous slender filiform costae, which 

 bifurcate on the upper part of the shell, becoming very re- 

 gular on the central portions, and again bifurcating irregular- 



