Plate X. 
All species illustrated on this plate are from the Cincinnati Group, and all the 
figures, unless otherwise marked, are magnified twenty diameters. 
Page. 
Figs. rand 2. Primitia centralis, n. sp... 5.1. 130 
I, a left valve of the wider form from the Utica shales horizon ; 
2a, 26 and 2c, side, anterior and dorsal views of a left valve of 
the narrow variety that occurs near the top of the hills at Cov- 
ington, Ky. 
Figs. 3 and 4. Primitia impressa, n-: Sp... 4) 3 are lee Be cey 
Two right valves, illustrating slight differences, with anterior and 
dorsal views of each. Savannah, Ill. . 
Figs. 5 and 6. Primitia cincinnatiensis, (S. A. Miller)....... 132 
5@ and 54, left side and dorsal views of a complete carapace, differ- 
ing in several points from the typical form of the species ; 6a and 
64, right side and anterior views of an average example ; 6c, inte- 
rior of a right valve. 
Fig..7. Primitia medialis, nosp...). 0.7.3 132 
7a, left valve, showing the narrow ridge in the sulcus; 76, end 
view of same. 
Fig. 8. Primitia rudis,-n.sp. ) 0 oh er 136 
8a, right valve ; 84and 8c, posterior and ventral views of same. 
Fig. 9. Primitia glabra, n.sp oo. 4.0) > 2 134 
ga, left valve; 94 and gc, posterior and dorsal views of same. 
Fig. 10. Aparchites oblongatus, m. sp...) 2 see ry 
10a, 104 and Ioc, right side, ventral and posterior views of a com- 
plete carapace of this species. 
Figs. 11 and'12, Primitia nodosa, n.-sp. 5-9-4 134 
21a, right valve of the typical form, x 20; 114, dorsal outline view 
of same, x 35; lower beds at Cincinnati, Ohio; 12a, right valve 
of the short variety (in which the ventral node is not developed), 
that isso far known only from strata exposed near the tops of the 
hills at Cincinnati; 124, posterior end of same. | 
Hig. 17. Placentula mareimatay 0. Sp... a eee Were: 124 
13a, 134 and 13¢, lateral, dorsal and anterior views of a right valve 
of this species. 
Migs, Placentula inormmatayyn Spy en Pee 124 
14a, 140, lateral and anterior views of aright valve, x 18. 
Fig. .15.° Beyrichia (? Primitia) parallela, Ulricha [25 
15a, left valve; 154, dorsal, 15¢, anterior, and 15d, posterior views of 
same. Richmond, Ind. 
Fig. 16. Pontocypris (2) iWlinoisensis, m. sp.) 2) aa 107 
16a, 164 and 16c, three views of a valve of this species, from 
Savannah, I[Il. 

