CLA>ss IV GASTROPODA 579 



Family 5. Philomycidae Gray. 



Slugs someiühat similar to Arionidae, hut the mantle Covers the entire U2)per surface 

 of the body. Ä shell is completely ahsent ; hence no fossil forms are hnown. 



Superfamily 4. ELASMOGNATHA Mörch. 



Jaio vnth a strong squarish process of attachment ahove. 



Family 1. Succineidae Albers. 

 Shell thin, ovate^ consisting of few whorls. 



Fig. 1097. 



Succinea Pfeiffer (Fig. 1097). Shell thin, ovate, amber-coloured, SiuTinea peregHna 

 transluceiit, witli short spire and large body whorl. Outer margin cene;' Tuchoritz, 

 of apertiue sharp. Tertiary and Recent ; abundant in the Loess, Bohemia. 



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 thtee-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, Bhaphistoma, Pleurotomaria ; a number 

 of Pteropods with some doubtful forms {Hyolithes, Hyolithellus, SaltereUa, 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 Plemvtomariidae, Euomphalidae and B eller ophontidae) predominate ; and 

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

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

 occurring here is Subulites, 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 separated as 

 at present. 



During the Ordovician and Silurian, Gastropods increased perceptibly 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 Opisth'obranchs, and also a scattering representation of Ctenobranchs 

 (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 

 Rhipidoglossa reached the acme of their development {Pleurotomariidae, Tur- 

 hinidae, Nerilopsidae, Neritidae); and among the Ctenobranchs, the families 



