582 ' MOLLUSCA phylum vi 



Pyramidellidae Nerineidae, Purpurinidae, TurriteUidae and Aporrhaidae multi- 

 plied in a great variety of forms. 



The Cretaceous witnessed a decided increase among the siphonostomous 

 Ctenobranchs, and in the Tertiary this branch asserted itself as the dominant 

 type of Gastropods, surpassing all other families in point of numbers, and 

 gradually acquiring the aspect of living genera and species. The Nerineidae, 

 Pyramidellidae and Aporrhaidae, which played such a prominent role along 

 with the Rhipidiglossa during the Mesozoic era, became in part extinct in the 

 Tertiary, and the remainder entered upon their decline. The great majority 

 of Eocene and Oligocene genera are still living, but the species have with 

 very few exceptions become extinct. During the Miocene, more species made 

 their appearance which are still in existence, and of the Pliocene species, 

 between 80 and 90 per cent are represented in the Recent fauna. 



The geological history of the Pulmonata is remarkable. Thalassophilous 

 Siphonariidae are first met with in the Devonian, where they are very sparse. 

 Land snails (Archaeozonites, Pyramidula, Dendropupa) were initiated in still 

 smaller numbers during the Carbon iferous; but not until the boundary between 

 the Jura and Cretaceous is reached do we find any traces of fresh-water snails. 

 We meet them first in the Purbeck. In the Wealden, and Cretaceous gener- 

 ally, both land and fresh-water Gastropods are quite abundant ; they became 

 highly developed and widely distributed during the Tertiary, attaining, in fact, 

 a differentiation nearly equal to that exhibited by the corresponding Recent 

 forms. 



The successive approximations to present conditions among Gastropod 

 faunas have not been confined to the production of forms simulating more and 

 more those now living ; they include also the gradual demarcation of existing 

 geographical provinces. Mesozoic Gastropods are too dissimilar in their 

 general characters to admit of a close comparison with modern faunas ; but as 

 early as the Eocene resemblances to modern forms are observable, and a certain 

 correspondence is to be noted with Gastropods now inhabiting somewhat 

 warmer zones. 



The Eocene faunas of Europe, North America, Asia and Northern Africa 

 share a great many genera in common, and have numerous others which are 

 vicarious. A very difFerent aspect is presented by the Eocene fauna of 

 Australia, New Zealand and South America, where we find the evident fore- 

 runners of forms now inhabiting the southern portions of the Atlantic and 

 Pacific Oceans. 



Still more intimate is the relationship existing between the fossil land and 

 fresh-water Gastropods and their descendants on the several continents. It 

 has been observed that Miocene faunas bear a decidedly tropical stamp. On 

 this account European and American forms from the inland Miocene deposits 

 bear some resemblance to the Recent faunas of the Azores and the West 

 Indies, as well as to the land and fresh-water Gastropods inhabiting the colder 

 latitudes of Europe and Asia. Only as recently as the Pliocene did each 

 geographical province come to assume its present distinctive features. 



In general, the stratigraphic sequence of Gastropod groups corresponds 

 closely with the zoological order, the most generalised forms appearing first, 

 the more specialised later. Beginning with the two-gilled Rhipidoglossa and 

 the Docoglossa, f ollowed by the single-gilled Rhipidoglossa, Opisthobranchs and 

 taenioglossate Ctenobranchs, the series leads to the Rachiglossa in later Mesozoic, 



