PLATE 1 . 



Page. 



Fig. 1. Cybena Dakotensis 15; ' 



1, a. Dorsal view of an internal cast. • 



1, b. Posterior view of same. 



1, s. Side view of a larger specimen of the shell. 



1, d. Dorsal view of another specimen of the two valves partly opened. 



1, c. Side view of another smaller specimen. 



1,/. Side view of a small internal east. 



I'll ;. 2. Arcopagella ? MACRODON TA 2 "2 



An eroded internal cast of the type-specimen, apparently the right valve. [The impres- 

 sions of the lateral teeth are not near long enough in the lithograph.] 



Fsg. 3. Piiarella Dakotensis 251 



A cast of the two valves opened out (type-specimen). 



Fig. 4. Cast of the burrow of some unknown mollusk. 



Fig. 5. Margaritana Nebrascensis 114 



5, a. Side view of the large, nearly entire, type-specimen. 

 5, b. A dorsal view of same. 



5, c. A side view of a smaller internal cast of same species. 

 Fig. 6. Tkigonarca Siouxensis y ~ 



A cast of the interior of the left valve (from the original typical figure by II. & M.). 



Fig. 7. Mactra Siouxensis 206 



7. a. A side view of the middle and posterior portions of the shell, showing surface-stria;, etc. 

 7, b. A side view of an internal cast, showing muscular and pallial impressious. 

 7. c. A posterior eud view of the two valves united; taken from an artificial cast made in a 

 natural mould of the shell left in the rock (both being type-specimens). 



