206 FRESHWATER FAUNA 



Dreissenas and Melanopsids (Figs. 72, 73). Both are 

 poorly represented in rocks of an earlier period, Melanopsis 

 in the middle Cretaceous, Dreissena in the Oligocene (D. 

 unguiculus of the Headon Hill beds), but now they burst 

 out into a sudden and exuberant efflorescence ; some of 

 their forms, worthy to rank beside the robuster kinds of 

 sea-shells, are shown in Fig. 73. A second, but less 

 luxuriant, expansion took place in the middle Pliocene, 

 and to-day these forms are richly represented in the 



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FIG. 73. Melanopsidse from the Miocene and Pliocene 

 beds. The figures in this series are about two- 

 thirds natural size. 1. Melanopsis pygmcea. 2. 

 M, Bouei. 3. M. cetolica. 4. Melanoptychia 

 Bittneri. 5. Melanopsis Heldreichi. G. M. Vindo- 

 bonensis. 7. M. Gorceixi. 8. M. Proteus. 9. If. 

 Martiniana. After Neuniayr, " Erdgeschichte." 



Ohio basin of North America and some of the rivers of 

 China. 



The freshwater beds which succeed the Pontic deposits 

 are the Levantine, corresponding approximately with the 

 marine middle Pliocene in age. They are distinguished 

 by their rich variety of freshwater shells, and especially by 

 numerous forms of Paludina. Neumayr in a remarkable 

 study of the latter has been able to trace the evolution of 

 a series of species which, commencing with a form very 

 similar to that of our own rivers, that is, with smooth 



