502 MOLLUSCA SUB-KINGDOM VI 
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, followed by the single-gilled Rhipidoglossa, Opisthobranchs, and 
taenioglossate Ctenobranchs, the series leads to Rachiglossa in the later Mesozoic, 
and culminates in the great increase of rachiglossate and toxoglossate families 
in Tertiary and Recent times.! (See tables, pp. 500, 501.) 
Class 5. CHPHALOPODA.’ 
Head sharply defined in recent forms, except Nautilus. Foot transformed into a 
[? Grateful acknowledgments are due to Professor Henry A. Pilsbry, of the Philadelphia Academy 
of Natural Sciences, for his revision of the preceding Gastropod chapter. The difliculty of 
adapting a strictly zoological classification, based largely upon the anatomy of the soft parts, to the 
practical needs of the palaeontologist, is strikingly illustrated by the class of Gastropods. A 
revolutionary, or even extreme course has been avoided ; and it is hoped that the system herein 
Bdoyied will be found to possess some practical advantages. —TRANS. ] 
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