50 



Cyclas. The Limnaea is not distinguishable from a Cuban species^ noT* 

 from L. umbilicata of Massachusetts. The Physa is probably P. 

 Soverbyana, of Cuba also. The Ancylus is not distinguishable from 

 A. obseurus of Pennsylvania. Ampullaria fascial a is said to occur in 

 several other parts of tropical America. But these four are genera 

 in which there is a great paucity of specific characters. The remain- 

 ing species have not been identified with those of other places, unless 

 Planorbis affinis be identical with P. lentus. In some localities, sev- 

 eral of the fresh-water species occur abundantly ; bnt these favor- 

 able stations are not numerous. Streams are very frequent in many 

 parts of Jamaica, but they are mostly mountain torrents, which rarely 

 present favorable stations for Mollusca. 



It has been observed by naturalists, that in tropical Zoology, while 

 the number of species is much greater than in northen regions, the 

 number of individuals is much less. This is eminently true of birds 

 and fishes, and of some other classes. Perhaps this generalization 

 does not hold true of any entire class of Mollusca, and if it be ap- 

 plied to the terrestrial Mollusca of the Western Hemisphere, the re- 

 verse is remarkably evident. In Jamaica, many of the terrestrial 

 species exist in a profusion to which there is no parallel in the tem- 

 perate and northern regions of Korth America. 



A comparison of the number and of the distribution of the marine 

 and of the terrestrial species, suggests an interesting conclusion. If 

 the same general fact is true in all the larger islands, viz. : that 

 the number of terrestrial species iu each island is from one third to 

 one half the number of the marine species, while 5 per cent, of the 

 former and 75 or 80 per cent, of the latter ar« common to two or 

 more islands, — it follows that the total number of terrestrial species in 

 the Antilles must exceed that of the marine species. 



With this extremely local distribution of the terrestrial Mollusca 

 in the West Indies may be associated the great fact of their geologi- 

 cal history — that these islands have, since the later Tertiary periods, 

 been in the process of elevation,— that they are the harbingers of a; 

 future continent, unlike the groups in the Pacific, which are the re- 

 mains of ancient continents. Coincident with these two general facts 

 the West in Indies is also a t]\ird, — that their coral reefs are all fring- 

 ing, and that coral lagoon islands are warning. 



The name Cyclostoma solidum is pre-occupied by Menke. The 

 species, which was described under this name on page 7, may there- 

 fore receive the name of C. Jatanum Ad. 



