1911. ] NATURAL SCIENCES OF PHILADELPHIA. 117 



VARIATION IN SOME JAMAICAN SPECIES OF PLEURODONTE. 

 BY AMOS P. BROWX. 



C. B. Adams, in his Contributions to Concholoyi/, No. 1 1 . published in 

 1852, gives some " Hints on the Geographical Distribution of Animals," 

 in which he points out that "each species occupies one geographical 

 area only; but, inasmuch as natural types are of all grades of value, 

 the difficulty of discriminating species is great. The difference 

 between some types which inhabit distinct areas is slight and in some 

 cases perhaps impossible to demonstrate; nevertheless, they should 

 be regarded as distinct species. Still it may occur that exceptions 

 to this rule exist, and frequently a number of pairs of such analogues 

 from distinct areas may be so arranged that the amount of difference 

 between each two shall successively diminish from species that are 

 very distinct to species that are scarcely distinguishable, and at last 

 the series shall terminate in two forms quite indistinguishable from 

 each other; that is, in one species. These areas of species vary from 

 a few miles to several thousand miles. Areas of insular terrestrial 

 species, excepting those with the power of flight, do not usually exceed 

 the islands they inhabit, and where the islands are separated by 100 

 miles or more of water examples of species common to two or more 

 are rare." 



In a previous article, published in the Contributions, No. 10, 1851, 

 "On the Nature and Origin of the Species of Terrestrial Mollusca of 

 the Island of Jamaica," he says: "Among the terrestrial shells, 

 typical forms exist in great profusion. These forms are of every 

 conceivable grade of value, from varieties up to genera and families. 

 They have also a determinable geographical distribution. . . . The 

 island in this respect is a miniature continent." In considering the 

 nature of the species, Adams remarks: "Our first conclusion is this: 

 that in many groups the species are distinguishable by types only 

 and not by well-defined limits. This may be illustrated by a figure. 

 in which species are represented by circles, many of which are in 

 contact and whose areas are sprinkled irregularly with dots which 

 represent the varieties. One central dot represents the type of the 

 species. ... On the boundaries of the species we find varieties 

 which closely resemble their neighbors in the adjacent species, while 



