250 Miscellaneous. 



an associated species, Pvpa incolata, "White, to Leucocheila. Mr. 

 Meek referred his Pupa Leidyi doubtfully to Holvspira. It is from 

 the Bridger strata of Wyoming. 



Only one species of the Succininse has yet been discovered in any 

 of the strata here considered, namely Succinea papillispira of the 

 Green-River strata of Wyoming. This is plainly referable to 

 Brachyspira. 



The Unionidse of the fossil molluscan fauna, herein discussed, are 

 found to have become differentiated to a remarkable extent, espe- 

 cially during the Laramie epoch. An exceedingly interesting and 

 suggestive fact in connexion with this differentiation is that the 

 subordinate types are largely identical in character with some of 

 those which are now living in the waters of the Mississippi river- 

 system, and which are recognized by malacologists as distinctively 

 North-American types. Illustrative of this relation of the fossil to 

 the recent forms, the following parallel lists are presented, those of 

 the left-hand column being a part of the fossil species now known 

 in the Laramie strata of Wyoming and Utah, and those of the 

 right-hand column being the living species of the Mississippi river- 

 system which are selected as their respective type congeners. 



Unio propheticus, White. Unio clavus, Lamarck. 



proavitus, W. ridibundus, Say. 



gonionotus, TV. niultiplicatus, Lea. 



- holmesianus, TV. — — apicnlatus, Say. 



Oouesi, W. complanatus, Solander. 



Endlichi, W. ■ gibbus, Barnes. 



brackyopisthus, W. circulus, Lea. 



Still other examples might be given of close resemblances be- 

 tween fossil and recent forms of Unio ; but these suffice to suggest 

 in a very forcible manner that the Unionine fauna of the Mississippi 

 river-system is genetically related to that of the Laramie period. 

 It is true that in the Laramie fauna there are certain minor types 

 of Unio which are not so closely like any living forms as those are 

 which have been cited, and that close congeners of certain living 

 types have not been discovered among the fossil forms ; but these 

 facts do not necessarily affect the legitimacy of the conclusion that 

 the living has genetically descended from the fossil fauna. A like 

 conclusion is also reached with reference to the pulmonate gaste- 

 ropods, which have already been discussed ; but in view of the 

 magnitude of the physical changes which have taken place since 

 the close of even the latest epoch here considered, the survival of 

 the types of the branchiferous Mollusca, and their transference from 

 lacustrine to fluviatile waters, is a most remarkable circumstance. 



Reviewing the collections which represent the fossil faunas herein 

 discussed, so many familiar forms are seen that it is difficult to 

 realize the fact that a large proportion of them, including those 

 especially which have been mentioned by name in this article, were 

 living contemporaneously with the last of the Dinosaurs. Yet 

 such is the fact, and the shells of the former are often found com- 

 mingled with the bones of the latter. What were the successive 

 steps in the history of the transmission of these types from that 

 remote time to the present we are unfortunately without the means 

 of knowing with certainty, because of the remarkable paucity of 

 mnllnsran remains in all the denosits of the great interior region 



