266 



MOLLUSCA. 



to a time anterior to the Carboniferous. The Pond-snails, be- 

 ing exclusively confined to fresh water, are only known as fossils 

 in fluviatile and lacustrine deposits, and they are exclusively 

 Secondary and Tertiary, not being known in the Palaeozoic 

 period. The Pulmonifera are divided into the two orders of 

 the Inoperculata and Operculata, according as the shell is des- 

 titute of an operculum, or is provided with this apparatus. 



ORDER IV. INOPERCULATA:^/^// not provided with an 

 operculum. 



FAM. i. HELICID^: : Shell well developed, capable of con- 

 taining the entire animal. With the exception of Pupa and 

 Zonites (the last a sub-genus of Helix), all the Helicida belong to 

 the Tertiary and Recent periods. As they are all terrestrial in 

 their habits, they are necessarily of rare occurrence as fossils, 

 occurring chiefly in fluviatile and lacustrine deposits. The two 

 genera above mentioned have been found in the Coal-Measures, 

 and are the oldest forms of the group. The chief fossil genera 

 are Helix, Bulimus, Achatina, Pupa, and Clausilia. 



In the genus Helix are the ordinary Land-snails (fig. 236), 

 in which the shell is conical, sometimes depressed, or some- 

 times discoidal ; the aperture transverse, crescentic or rounded, 

 and the columella perforated or imperforate. The Land-snails, 

 with one exception, are all confined to the Tertiary and Recent 

 periods. The exception to this statement is \he Zonitcs priscus 

 (fig. 236), discovered by Dr Dawson in the Coal-Measures of 





I 



Fig. 236. Zonites (Contilus) priscus (after Dawson). a Specimen enlarged twelve 

 diameters; b Sculpture, magnified. Coal-Measures, Nova Scotia. 



Nova Scotia. This is a true Land-snail referred to Zonites or 

 Conn/us, a sub-genus of Helix itself. 



In Bulimus the shell is turreted or oblong, the columella 

 generally simple, and the outer lip usually expanded and 

 thickened. In the nearly allied genus Achatina the columella 



