SUB-ORDERD RANGE AND DISTRIBUTION OF THE GASTROPODA 499 
but scanty information regarding the disposition of the soft parts ; neverthe- 
less, there are good, although purely theoretical reasons for supposing that 
the Lhipidoglossa 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 (Scalaridue, 
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, Lhipidoglossa, a few 
Docoglossa and Opisthobranchs, and also a scattering representation of Cteno- 
branchs (Capulidae, Pyramidellidae, 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 
Lhipidoglossa reached the acme of their development (Pleurotomaridae, Tur- 
binidae, 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 Lhipidoglossa 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 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 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 
