434 WEST INDIAN MOLLUSKS— SIMPSON. vol. xvii. 



Antilles, and has a few representatives on tlie mainland, the Bahamas, 

 and the Lesser Antilles. 



All the species of the subgenus PtychoeochUs agree very closely in 

 their corrugated shells and the cliaracter of the opercula; and this 

 group, together with the typical Sagdas and Pleurodontes, are confined 

 to the island of Jamaic;a. It is not unreasonable to suppose tliat during 

 the period of general elevation certain forms from the widely distributed 

 genera of land and freshwater moUusks crossed over to the Greater 

 Antilles from the continent, that such genera as GJandina, Streptostyla, 

 and others whose metropolis is on the mainland also migrated across, 

 and that species of a number of genera whose greatest development 

 is in this archij)elago spread out and reached the shores of America. 

 Most of the subordinate groups of Glandina and Streptostyla, and several 

 of those of Cylindrella were then in existence, for we find their species 

 to-day alike on the continent and the different islands of the archipelago. 

 During the subsidence, which must have been gradual, Jamaica was first 

 separated from the rest of the Greater Antilles, and between the time 

 of separation and the date of laying down the Bowden marl it is probable 

 that the typical Pleurodontes and PtychoeochUs were developed from 

 some less differentiated, ancestral stock. The separation of Cuba, which 

 occurred sometime after that of Jamaica, gave rise to the special Cuban 

 groups, or no doubt to such of them as are dominant and abundantly 

 represented on the island; while Haiti and Puerto Rico, being longer 

 united, have a much more closely related fauna. 



In his catalogue of the terrestrial and tluviatile mollusks of Haiti* 

 Crosse divides the island into four subregions — one on the north, taking 

 in the Sierra de Monte Christi; another .south of this, extending from 

 the Mole St. Nicholas through the island to Cape Engailo; a third 

 embracing the southeast peninsula, and the fourth situated between 

 the arms of the Y, and he remarks significantly : 



It is remarkable that the purely geographical considerations on which some 

 authors regard Haiti as a link that formerly nuited the four islands are confirmed 

 and corroborated by the existence in each of the four regions of a kind of small 

 malacological fauna, independent of species which are scattered throughout the 

 island and which comprise the common fauna. 



Every species of Colobostyhis known on the island, the group Thau- 

 masia of the genus Gylindrella, and the representatives of Yendreysia, 

 Stoastoma, and Lncidella, all of which have their metropolis in Jamaica, 

 are found in the southwest peninsula, while the great Helices of Cuban 

 groups are met with in the northwestern arm of the island, and the 

 species of the east end show an alliance with the forms of Puerto Rico. 



In the Miocene silex beds of Tampa, Florida, there have been found 

 a number of land shells which i^robably belong to the same fauna as 

 that which existed during that epoch in the Greater Antilles. These 

 consist of six Helices of the section Plagioptycha, a group at i)resent 



*.Jour. de Conch., xxxi, 1891, pp. 195, 197. 



