162 PROCEEDINGS OF THE ACADEMY OF [Feb., 



collected at the King Edward woods at an elevation of less than 2,000 

 feet, probably nearly one-third nearer the sea level than the Ridge 

 near Lincoln colony. But the diminution in size in passing from the 

 valleys and bases of the hills to the tops, which accompanies the rise 

 of the spire, is an undoubted fact, although diminished atmospheric 

 pressure is not here the controlling factor. That it should ever be a 

 controlling factor does not seem likely, as the variation from day to 

 day in barometric pressure at a given point may, in this latitude, at 

 least, amount to what would be equivalent to a difference in elevation 

 of 1,000 feet or even as much as 1,500 feet inside of 24 hours. The 

 diminution in size and the rise in spire are independent of absolute 

 altitude above sea level, but are noted in passing from the bottom to 

 the top of one hill. There is in P. a. goniasmos a direct connection 

 between these two characters, the size and the height of spire vary 

 inversely. The cause of the diminution in size may well be the cause 

 of the rise of the spire also. From the observations that I have made 

 upon these Jamaica mollusks this diminution in size, which accompanies 

 the rise in the height of the spire, appears to be controlled mainly 

 by the distribution of moisture. Variations in the conditions of 

 moisture may occur quite independently of the altitude, and I expect 

 in a subsequent paper to discuss this influence in connection with 

 some other species where altitude is not a factor, and where the dis- 

 tribution of moisture is not controlled by the altitude nor by the 

 topography. 



On the hill-tops, as has been pointed out, the amount of moisture 

 found in the ground, in the leaf cover in which these Pleurodonts live, 

 is less than on the slopes of the hills, and, from the wooded valleys and 

 gullies to the hill-tops, this moisture in the leaf cover steadily dimin- 

 ishes, so that, as has been shown, the available food for the molluscs 

 diminishes from the lower ground to the hill-top. Quite as important 

 is the fact that the hill-tops are only moist and the conditions of 

 moisture favorable for the growth of the mollusks at intervals fol- 

 lowing rains. In the gullies and wooded valleys the supply of mois- 

 ture and food is continuous, on the hill-tops it is intermittent. 

 The forms living in the hollows and lower ground at any place are, 

 therefore, continuously supplied with the necessaries of life, while 

 those living on the hills are supplied intermittently. Growth is 

 continuous in the lower-ground forms, but it proceeds with many 

 stoppages on the hill-tops. In a given period of time, therefore, the 

 lower-ground forms can grow on more days, perhaps often on twice as 

 many days or even more, than those living in the higher slopes and 



