THE AGE OF THE LIGNITIC FORMATION BY ITS FAUNA. 29 



the upper valve, supposing it to be an Oyster. If these depressed specimens 

 in our collection are opposite valves to the convex ones, then the shell would 

 neither be an Ostrea nor an Anomia, but would almost certainly fall into 

 Morris and Lycett's genus Placunopsis, which, so far as known in Europe, is 

 a Jurassic group. Consequently, if our shell should fall into that genus, it 

 would, when viewed in connection with its associates and all the other known 

 facts, furnish a strong argument in favor of the formation being at least as old 

 as the Cretaceous. There are good reasons, however, for believing that these 

 depressed specimens, as well as the convex ones, are all upper valves of the 

 same shell, only modified in convexity by accidental circumstances of station, 

 as their slight obliquity, as seen, for instance, in a look at the interior of both, 

 is found to be in the same direction instead of the reverse, as would be the 

 case if they were opposite valves of the same shell; while among thousands 

 of specimens no example of a depressed and a convex valve united has been 

 seen, nor have any been found that would come near fitting together.'' 



"On the other hand, the Corbiculas are decidedly Tertiary in their spe- 

 cific affinities, as well as in tlieir subgeneric; C fracta, for instance, and 

 C. crassatelliformis, from the Hallville mines, being very closely allied to 

 Paris Basin Tertiary forms, the first-mentioned species being the type of a 

 subgenus, so far as known, peculiar to the Tertiary elsewhere. The same 

 may also be said of C. cytheriformis, which also seems to belong to a group 

 {Velorltina) peculiar to the Tertiary in Europe." 



"But the most surprising fact to me, supposing this to be a Cretaceous 

 formation, is that we found directly associated with the reptilian remains at 

 Black Buttes a shell I cannot distinguish from Viviparus trochiformis, origin- 

 ally described from the Lignitic formation at Fort Clarke, on the Upper Mis- 

 souri, a formation that has always been regarded as Tertiary by all who have 

 studied its fossils, both animal and vegetable. The specimen mentioned 

 does not show the aperture nor all the body-volutions, but, as far as can be 

 seen, it agrees so exactly witii that very peculiar species in size, the form 

 and proportion of its volutions, the slopes of its spires, its surface-markings, 

 the nature of its suture, and, in fact, in every respect, so far as can be seen, 

 that I have scarcely any doubt of its identity with the same." 



In resuming his remarks, Prof Meek further states, p. 461, § 7 to 9: — 



"That on the one hand, two or three of its species belong to sections or 

 subgenera (Leptestkes and Fe/or2>ma) apparently characteristic of the Eocene 



