OF DENISON UNIVERSITY. 
15 
a. Surface with over 50 rounded ribs, hinge with about six ob- 
lique spines. 
5. C. PULCHELLA. 
b. Fewer striae, which are sharply raised, four oblique spines. 
6. C. SCITULA. 
The typical form of C illinoisensis occurs at the very top of the series. A 
closely allied form, perhaps C. multicosta appears about seventy feet below con- 
glomerate. C. logani is ubiquitous while C. tumidus and C. scitula belong up- 
on the same horizon as C. multicosta. C. mesoloba must be excluded from the 
list. 
Genus Productus. 
I. Species of large size (over ^ inch.) 
A. Marked by fine, wavy, fasciculate radiating costae. 
1. P DUPLICOSTATUS. Win. 
II. 
B. Marked by strong, radiating (and concentric) costae. 
Bi. Strongly pustulose. 
(?) 2. P. NEBRASCENSIS. 
B2. Not Strongly pustulose. 
* Length over one inch, 
t Coste distant, 
3. P. RARICOSTATUS. Her. 
(=P. dolorosa. Win?) 
ft Costae closely set, 
a. Ventral valve tumid, strongly arched over the hinge. 
4. P. SEMIRETICULATUS. 
b. Ventral valve moderately tumid, less strongly arch- 
ed over the hinge. 
5. P. NEWBERRYI. Hall. 
**. Length less than one inch. 
a. 
b. 
With very coarse and irregular costae, very strongly 
arched" 
6. P. ARCUATUS. Hall. 
Striae finer and more regular, less strongly arched. 
7. P. FLEMINGI. Var. 
Of small size, (mostly under ^ inch.) 
A. Marked only by fine dichotomizing striae. 
8. P. GRACILIS. Win. 
B. Marked with fine concentric striae and irregular coarse pustu- 
lose ribs. 
a. Very tumid with numerous spines on the ears, not strongly 
pustulose. 
