412 MOLLUSCA. 



shell than Corbula, which it represents, and Neritina than 

 Nerita. I have found a species of Pkolas in the fresh- 

 water rivers of Borneo, living in dead trunks of trees, 

 which is partially covered with a thick brown epidermis. 



It is a curious fact that the nearer mollusks live 

 to the sea-water, the more dense their shells generally 

 become. This may be noticed in those species of 

 Auricula and Melampus, found among the loose stones 

 on beaches ; and among the Korean Islands, I have found 

 a Cydostoma, in heaps of stones, near the sea, of a very 

 compact appearance, compared with the terrestrial species. 

 The Cyclotrema and Scalaria, their marine analogues, 

 are yet more calcareous and dense in their structure. 

 Among the Philippines, I have observed some auriculari- 

 form MitrfB crawling about the stones, which the receding 

 tide had left exposed, in the manner of the Quoyia and 

 certain species of Planaxis. These Mitrce have an 

 epidermis, and are hardly of so dense a nature as other 

 members of the family. The exception to the foregoing 

 rule is to be found in those pelagic animals, in which the 

 extreme lightness of the shell constitutes their best 

 security ; for the ocean may toss them in its fury, but, 

 unless a foreign body interpose, their tenuity saves them 

 from being injured. 



While residing at Brunai, I had an opportunity of 

 examining the celebrated Upas-tree which grows on the 

 banks of the river opposite the city, and a short ac- 

 count of it is given in the body of the work ; a few 

 notes which I shall here add, may not, however, be 

 deemed altogether void of interest. 



