512 GEOGRAPHICAL ZOOLOGY. [part iv. 



Family 20.— DENTALIAD.E. (1 Genus, 50 Species). 



Distribution. — The genus Dentalium is found in the North 

 Atlantic, Mediterranean, West Indies and India. There are 

 125 fossil species, found in various formations as far back as 

 the Devonian in Europe and in Chili. 



Family 21.— CHITONID,E. (1 Genus, 250 Species). 



Distribution. — On rocky shores in all parts of the world. 

 There are 37 fossil species ranging back to the Silurian 

 period. 



Order IL—PULMONIFERA. (" Terrestrial Molluscs.") 



The Land and Fresh-water snails are so important and exten- 

 sive a group, and their classification has been so carefully 

 studied, that their geographical distribution is a subject of much 

 interest. The range of the genera will therefore be given in 

 some detail. For the Helicidse I follow the classical work of 

 Albers — Die Helicien, Von Martens' Edition (1860) ; and for 

 the Operculate families, Pfeiffer's Monogra/phia Fneumonopo- 

 morum Viventium, 2nd Supplement, 1865. The number of 

 species is, of course, very considerably increased since these 

 works were published (and the probable amount of the increase 

 I have in most cases indicated), but this does not materially 

 affect the great features of their geographical distribution. 



Family 22.— HELICID^. (33 Genera, 3,332 Species) (1860). 



General Distribution. — Universal. 



The Helicidse, or Snails, are a group of immense extent and 

 absolutely cosmopolitan in their range, being found in the most 

 barren deserts and on the smallest islands, all over the globe. 

 They reach to near the line of perpetual snow on mountains, and 



