6 GASTEROPODA. 



the families of the Tornatellidce, Bullidce (Bubble-shells), Aphj- 

 siadce (Sea-hares), Pleurobranchiadce, and Phijllidiadce. 



Division b. Nudibranchiata. — Animal destitute of a shell in the 



adult condition. Branchice external, on the back or sides of the 



hodxj. This division includes the various naked Gasteropods 



commonly kno-mi as Sea-lemons and Sea-slugs. 



Order III. Nucleobranchiata, or Heteropoda. — Shell present 



or absent. Animal free-swimming and oceanic, "with a fin-like tail 



or flattened ventral fin. This order includes the two families of the 



Firolidm and Atlantidce. 



Section B. Pulmonifera. — Respiration aerial, by means of a pul- 

 monary chamber. 



Order IV. Ixoperculata. — Shell not provided with an oper- 

 culum. This order comprises the families of the Helicidce (Land- 

 snails), Limacidce (Slugs), Oncidiadce, Limnceidce (Pond-snails), and 

 Auriculid(B. 



Order V. Operculata. — Shell provided ■nith an operculum. In 

 this order are the families Cyclostomidce and Aciculidre. 



Distribution of the Gasteropoda in time. — As regards the 

 general distribution of the Gasteropods in past time, all the 

 families of the Prosohranchiata are known by fossil represen- 

 tatives. Of the Oiyisthohrancliiata the Tectibranchiate section 

 is tolerably well represented in past time ; but the section of 

 the Nudibranchiata, from the total absence of the shell, is not 

 known at all in the fossil condition. Both families of the 

 Heteropoda are represented by fossil forms. The Pulmonate 

 Gasteropods, from the fact that they either live on land or 

 inhabit fresh water, are necessarily not so fully represented 

 in past time as are the Branchiate Gasteropods. Still, nearly 

 all the families of the air-breathing Univalves have fossil 

 representatives. 



Taken as a whole, the Gasteropoda are represented in past 

 time from the Upper Cambrian rocks upwards. Of the 

 Braneliifera, the Holostomata are more abundant in the 

 Palseozoic period ; and the Sijjhonostomata predominate more 

 in the Secondary and Tertiary periods, attaining their maxi- 

 mum at the present day. The place of the carnivorous 

 Si2)honosto7nata in the Palseozoic seas appears to have been 

 filled by the Tetrabranchiate Cephalopods. The Branchiate 

 Gasteropods of fresh water are chiefly represented as fossils 

 by the genera Paludina, Valvata, and Ampullaria. 



