LIFE OX THE EARTH. 97 



Heteropoda. The symmetrically convolute Belle- 

 rophon, and its allies Cyrtolites and Porcellia, are 

 confined to Palaeozoic Strata. The recent Carinarise 

 are represented by one tertiary Italian fossil. 



Gasteropoda. The marine kinds all breathing 

 by gills and freely swimming in their embryonic 

 state by help of two ciliated fins, retractile with the 

 body into a convolute shell grow by constant laws 

 of development into shells of various forms, usually 

 spiral, though examples occur in which this character 

 is insensible (e. g. Patella). The most numerous order 

 both in the recent and fossil state receives the title 

 of Prosobranchiata, from the anterior position of the 

 gills. This character cannot be recognized in fossils, 

 but the forms of the shells are always sufficient to 

 identify the order. It may be divided into two sub- 

 orders : Holostomata with entire aperture to the 

 shell, Siphonostomata with the aperture notched or 

 canaliculate. The former are for the most part 

 feeders on vegetables, the latter, for the most part 

 carnivorous. The late Mr Dillwyn remarked the 

 prevalence of the siphonostomatous shells in the Ter- 

 tiary Strata, and the extreme rarity of them in older 

 strata. This curious generalization is found to be 

 of much importance in general views regarding the 

 succession of Molluscous life. 



No siphonostomatous shell has yet been found 

 R. L. H 



