120 PROCEEDINGS OF THE ACADEMY OF [Feb., 



•the shell or give a key to the mean divergence. The dimensions, 

 reduced to an average for each colony, may also be compared, and 

 when these are arranged in order geographically, on one diagram, they 

 give a view of the successive variations from station to station. 



I included in the specimens taken a considerable number of the 

 old "dead" or semi-fossil shells which are to be found in the soil and 

 in the "graveyards," as they may be called, the great accumulations 

 of old shells that are found at the bases of cliffs, in crevices and in the 

 little "cockpits." More of these should have been taken, for I found 

 on studying them in connection with the living forms of the same 

 colony that additional light was thrown upon these living forms. 

 But their importance was not fully recognized in the field, as each 

 day's collection was simply packed away for later study. They often 

 show much variation from the living forms, and, while many of these 

 "semi-fossil" specimens probably represent the form of the species 

 before the settling and clearing of the country, they also show the 

 amount of variation which may have taken place in a given locality. 

 While it is probably impossible to fix the time that has elapsed since 

 these forms were living, this was probably not great ; yet they give an 

 indication of how rapidly organisms may vary when living under 

 conditions that favor variation. Their study, too, may throw some 

 light upon similar variations that are noted in palseontological studies, 

 and Some of them may actually be fossils. 



In order that the conditions at each colony may be comprehended, 

 a description of this part of Manchester Parish and of the individual 

 colonies will be necessary (see map, fig. 1). 



Mandeville is located about the center of the parish of Manchester, 

 some 14 miles in an air line from the sea at Alligator Pond on the 

 south of the island of Jamaica and 35 miles in an air line from the 

 north coast. It is south of the "backbone," or main east and west 

 elevation of the island, which lies some 12 or 14 miles to the north. 

 It is on an elevated plateau which extends to the northwest of Mande- 

 ville, to Balaclava, Accompony, Ipswich and the "Cockpit Country." 

 The surface is very uneven, but the individual hills are not large nor 

 high; the small vallej^s are not very deep. Westward from Mande- 

 ville are seen several parallel lines of hills, running north-northwest 

 and south-southeast, and becoming higher to the westward until at 

 about 6 or 8 miles to the west the ground slopes abruptly down to the 

 Black River Valley and the level savannas of St. Elizabeth Parish, 

 which are but little above sea level — a drop of 2,000 feet. To the east 

 and northeast the ground slopes down to the valleys in which Wil- 



