758 GASTROPODA. 



are oceanic in habit, and are found swimming in the open sea, near 

 the surface, far from land ; but the majority of the marine Proso- 

 branchs and the Opisthobranchs are inhabitants of shallow seas, or 

 live between tide-marks. In depths beyond five hundred fathoms 

 the number of Gastropods is greatly reduced ; but a few forms are 

 found to inhabit depths of between two and three thousand fathoms, 

 or even more. The Pulmonate Molluscs, lastly, are exclusively ter- 

 restrial in habit, or live in fresh waters. In the latter case, the 

 animal either comes to the surface from time to time, for the pur- 

 pose of obtaining air, or, if the water be too deep to allow of this, 

 its pulmonary chamber is so far modified that it is enabled to 

 breathe the oxygen dissolved in the surrounding water. 



As regards their distribution in time, the Branchiate Gastropods 

 are necessarily more largely represented as fossils than the Pul- 

 monate forms. The record of the Prosobranchiates, owing to their 

 possession of a calcareous shell and their aquatic habits, is a very 

 full one. On the other hand, the Opisthobranchiates are but 

 imperfectly represented in past time, the entire group of the Nudi- 

 branchs being unprovided with a shell, and being therefore unknown 

 in the fossil condition. The oceanic group of the Heteropods (if 

 the Bellerophontidce be placed elsewhere) is only represented in the 

 latest Tertiary deposits ; while the Pteropods, also pelagic in habit, 

 occur as fossils in the older Palaeozoic rocks, but are singularly 

 absent from the greater part of the Mesozoic and Kainozoic for- 

 mations. Of the Pulmonate Gastropods, those forms which live 

 habitually in fresh water (e.g., the Limnceidce) are naturally more 

 largely represented in the fossil condition than the purely terrestrial 

 forms (e.g., the Helicidce) ; but the remains of the latter are not un- 

 common in deposits which have been formed under suitable 

 conditions. 



The Gastropods appear for the first time in the Upper Cambrian 

 deposits, from which a number of forms have been obtained, all 

 belonging to the holostomatous division of the Prosobranchiates, 

 the two principal genera being Murchisonia and Pleurotomaria. 

 The Upper Cambrian rocks have also yielded the oldest examples 

 of Hyolithes and its allies, which are usually regarded as belonging 

 to the Pteropoda. In the Ordovician and Silurian rocks are found 

 very numerous types of the Prosobranchiate Gastropods, all of 

 which — with the apparent exception of such forms as Subulites 

 and Euchrysalis — are holostomatous. The predominant groups 

 in these ancient deposits are the Euomphalidce, Pleurotomariidtz, 

 and Bellerophontidce. The Pteropods are represented by the 

 aberrant types Conularia, Hyolithes, and Tentaculites. In the De- 

 vonian, Carboniferous, and Permian formations the general character 

 of the Gastropodous fauna undergoes little change, the predominating 



