GASTEROPODA. 265 



genus range from the Lower Silurian to the Carboniferous 

 Rocks, and are, therefore, exclusively Palaeozoic. 



In Ecculiomphalus (fig. 235) the shell is very like that of 

 Cyrtolites, but the whorls are few in number, and are widely 

 separated from one another. The shell is thin, and the coils 



Fig. 235. Ecculiomphalus distant. Quebec Group (Upper Cambrian?) 



lie in the same plane. The species of this genus range from 

 the Lower Silurian to the Carboniferous, and have been com- 

 pared to Euomphali imperfectly rolled up ; but the true affini- 

 ties of the genus are doubtful. 



Lastly, the genus Porcellia includes shells which are com- 

 posed of many whorls coiled into a flat spiral. The whorls are 

 keeled, and the aperture has a dorsal slit. The species of this 

 genus are mainly Palagozoic, but range into the Trias. 



SECTION B. PULMONIFERA : Respiration aerial, by means 

 of a pulmonary chamber. The Pulmonifera include the ordin- 

 ary Land-snails, Slugs, Pond-snails, &c., and are usually provided 

 with a well-developed shell ; though this may be rudimentary 

 (as in the Slugs), or even wanting. In the Land-snails and 

 Pond-snails there is a well-developed shell into which the 

 animal can retire completely. The Slugs, again, have a merely 

 rudimentary shell which is completely concealed within the 

 mantle. The completely shell-less forms are necessarily wholly 

 unknown as fossils. The Slugs, with a rudimentary shell, are 

 only doubtfully represented in a fossil state, and that only in 

 the Tertiary Rocks. The abundance of the shell-bearing forms 

 as fossils depends mainly on the habits of the animal. The 

 Land-snails, being terrestrial in their habits, are, necessarily, 

 but sparingly represented as fossils, and they do not date back 



