SUB-ORDER D RANGE AND DISTRIBUTION OF Till: (JASTROPODA 499 



but scanty information regarding the disposition of the soft parts ; neverthe- 

 less, there are good, although purely theoretical reasons for supposing that 

 the Rhipidoglossa and Ctenobranchs were formerly not so widely separated as 

 at present. 



During the Ordovician and Silurian, Gastropods increased perceptibly in 

 the number of species, and a few new families were initiated (S!ea/ //'/'/ A//-, 

 Purpurinidae, Trochidae, Xenophoridae) but the faunal aspect remained on 

 the whole much the same as in the Cambrian, and no essential changes were 

 introduced during the remainder of the Palaeozoic. Accordingly, the Palaeo- 

 zoic Gastropod fauna may be said to be characterised by its general simplicity, 

 being made up principally of so - called Pteropods, Rhipidoglossa, a few 

 Docoglossa and Opisthobranchs, and also a scattering representation of Cteno- 

 branchs (Capululae, Pt/ramidellidae, Littorinidae). 



During the Jura-Trias, the large, thick-shelled varieties of Pteropod-like 

 Mollusks became extinct. But, on the other hand, various families of the 

 Rhipidoglossa reached the acme of their development (Pleurotomariidac, '/'///- 

 linidae, Neritopsidae, Neritidae) ; and among the Ctenobranchs, the families 

 Pyramidellidae, Nerineidae, Purpurinidae, Turritellidae, 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 Rhipidoglossa 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 almost 

 without exception become extinct. During the Miocene, a few 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 Pulmonafa 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 Carboniferous ; 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 species in common, and have numerous others which are 



