DISTRIBUTION OF THE MOLLUSCA IN TIME. 28T 



cula, Carychium. In the lower chalk of Europe many forms have 

 been discovered which resemble livino- American tj-pes : Pleuro- 

 cera, Lioplax, Goniobasis ; in the middle and upper chalk, C3^rena, 

 Melanopsis, Paludomus, Paludina, Melania, Grlandiua, Bulimus, 

 Physa, Cyclotus, Cyclophorus (?), and Leptopoma (?) abound, 

 mixed with extinct genera : Dejanira, Anastomopsis, Lychnus. 

 The genera of the chalk period w-hich still exist have in great 

 part a quite different modern distribution. 



"In the eocene of France, have been discovered Amphidromus, 

 Glandina, Cylindrella, Columna, Megaspira, Cyclophorus, Cras- 

 pedopoma, associated with Melanopsis, Melania, Pirena, gigantic 

 Physas, Cyrena, Unio, etc. The miocene and pliocene are 

 characterized b}^ numerous species of Helix, Pupa, Glandina, 

 Cyclostoma, Megalomastoma, Strophostoma, colossal Clausiliae, 

 Testacella, Parmacella, Yalenciennesia, Pyrgula, Fossarulus, 

 Lithogiyphus, Paludina, Unio, Dreissensia, Dreissenomj^a, etc. 

 The abundance of species and their variability was remarkable 

 during the deposit of the Paludina-beds." 



In America numerous land and fresh-water shells are found in 

 the strata ranging from the Cretaceous to Eocene, which can not 

 only be referred positively" to existing genera, but even to smaller 

 groups now existing ; for instance, there are 5 groups of Helix, 

 Planorbis 3 gr., Limngea 3, Physa 2, Pupa 2, Succinea and Unio. 



We cannot better conclude this short notice of fossil land and 

 fresh-water shells, than by a reference to the remarkable tertiary 

 deposits of Steinheim, which have caused much discussion 

 among palaeontologists. 



At Steinheim, in Wiirtemburg, in what was once the bed of 

 an ancient lake, pits have been dug, revealing a succession of 

 tertiary strata of clay, shell-sand and limestone, and, commencing 

 with several forms of one species of Planorbis, or with related 

 species, it matters little which view is adopted, the superimposed 

 strata show a gradual divergence from the primal types until 

 in the latest deposits some of these have become so altered as to 

 have more resemblance to turbinate Yalvatse than to Planorbis. 

 That the extraordinary changes here produced were the result 

 of extraordinary conditions, can scarcely be doubted, but it 

 cannot be denied that here the gradual change wrought in 

 specific characters has received an important practical demonstra- 

 tion. The latest forms are proved to have been evolved from the 

 earliest, for the whole history of the evolution is laid bare in the 

 series of strata through which the innumerable specimens of these 

 Planorbes are disseminated. It is believed that, in this case, the 

 deposition of the strata was rather rapid, and therefore no great 

 amount of time was required to make the transitions of form.* 



* The Genesis of the Tertiary Species of Planorbis at Steinheim (Wiirt- 

 emburg). By Alpheus Hyatt. Anniversary Memoirs of the Boston Soc. 

 of Nat. History, 1880. 



