GASTEROPODA. 245 



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

 the Firolidce and Atlantida. 



SECTION B. PULMONIFERA. Respiration aerial, by means of a pul- 

 monary chamber. 



ORDER IV. INOPERCULATA. Shell not provided with an oper- 

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

 snails), Limacidte (Slugs), Oncidiadtz, Limnceida (Pond-snails), and 

 Avriculidtz. 



ORDER V. OPERCULATA. Shell provided with an operculum. In 

 this order are the families Cyclostomidce and Aciculidee. 



Distribution of the Gasteropoda in time. As regards the 

 general distribution of the Gasteropods in past time, all the 

 families of the Prosobranchiata are known by fossil representa- 

 tives. Of the Opisthobranchiata 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 hot 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 

 Branchifera, the Holostomata are more abundant in the Palaeo- 

 zoic period ; and the Siphonostomata predominate more in the 

 Secondary and Tertiary periods, attaining their maximum at 

 the present day. The place of the carnivorous Siphonostomata 

 in the Palaeozoic 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. 



The Heteropoda are likewise of very ancient origin, having 

 commenced their existence in the Upper Cambrian deposits. 

 The genera Bellerophon, Porcellia, Cyrtolites, and Maclurea, 

 are exclusively Palaeozoic ; Bellerophina is found in the 

 Gault (Secondary), and Carinaria has been detected in the 

 Tertiaries. 



The Pulmonate Gasteropoda, as was to be anticipated, are 

 not found abundantly as fossils, occurring chiefly in lacustrine 

 and estuarine deposits, in which the genera Limnaa, Physa, 

 Ancyius, &c., are amongst those most commonly represented. 

 These, however, are entirely Mesozoic and Kainozoic. In 

 the Palaeozoic period the sole known representatives of the 



