CLASS IV GASTKOPODA 579 



Family 5. Philomycidae Gray. 



Slugs sometohit similar to Arionidae, hut the mantle covers the entire thpfer surface 

 of the body. A shell is completely absent ; hence no fossil forms are hnoivn. 



Superfamily 4. ELASMOGNATHA Morch. 



Ja%v ivith ct strong squarish process of attachment above. 



Family 1. Succineidae Albers. 

 Shell thin, ovate, consisting of few whorls. p^^, ^ 



Succinea Pfeiffer (Ficf. 1097). Shell tliin, ovate, amber-coloured, Snwinca jwregriim 



1 -,1 T \ • IT 1 1 1 T /-v J • Sandb. Lower Mio- 



translucent, with short spire and large body whorl. Uuter margin cene; Tuchoritz, 

 of aperture sharp. Tertiary and Eecent ; abundant in the Loess. Boliemia. 



Range and Distribution of the Gastropoda. 



Of all classes of Mollusks, the Gastropods exhibit the most manifold 

 variety. Beginning in the Cambrian, they acquire a very gradual increase 

 and distribution, and are at present enjoying their maximum vigour. There 

 exist probably over 20,000 Recent species, about three-fifths of which have 

 gills, the remainder being air-breathers. 



At the base of the Cambrian (Olenellus zone) are found such archaic 

 genera as Scenella, Stenotheca, Platyceras, Ehaphistoma, Pleurotomaria ; a number 

 of Pteropods with some doubtful forms (Hyolithes, Hyolithelliis, Salterella, Torel- 

 lella, etc.), which evince the great antiquity of the Aspidobranchs ; and forms 

 resembling the Capulidae. In the later Cambrian the Rhipidoglossa (represented 

 by the Pleurotonmriidae, Euomphalidae and Bellerophontidae) predominate ; and 

 associated with these are certain Pteropod remains, members of the Capulidae, 

 and a few genera probably referable to the Turbinidae. A notable genus 

 occurring here is Suhdites, which bears some resemblance to the Pyramidellidae, 

 and exhibits a distinct channelling at the base of the columella. 



Unfortunately the poorly preserved remains of Cambrian Gastropods afford 

 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 sejjarated as 

 at present. 



During the Ordovician and Silurian, Gastropods increased perce2:)tibly in 

 the number of species, and a few new families were initiated {Epitoniidae, 

 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 Paleozoic. Accordingly, the Paleo- 

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

 being made up principally of Pteropods, Rhipidoglossa, a few Docoglossa 

 and Opisthobranchs, and also a scattering representation of Ctenobranchs 

 {Capulidae, Pyramidellidae, Littorinidae). 



During the Jura-Trias, the hirge, 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 (Pleurotomariidae, Tur- 

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



