METAZOA MOLLUSCA. 229 



shell. This subject of color variation in a single species 

 is shown still better by Nerita peloronta Linn. (No. 495). 

 Here we have nineteen shells varying from a nearly white 

 shell spotted here and there with pink to a dark purplish 

 shell apparently banded and spotted with lighter shades. 

 The youngest stage in all the specimens is either white 

 or yellow, without spots or bands, and is smooth ; (y) 

 and (z) are in their natural state, while the others have 

 been boiled in caustic potash for about ten minutes, so 

 that these bright colors are not seen in the living ani- 

 mals ; (z) is an adult which is mounted to show the bright 

 red spot near the large tooth that has given the name of 

 Bleeding Tooth to this species, and also the operculum 

 which is in place and is deeply colored, (q) is an illus- 

 tration of albinism, the spot that is usually blood red 

 being nearly colorless. 



Neritina (No. 496, N. canalis Sowb.), is a genus of 

 especial interest since some of the species live in the 

 saltest of sea water, that of a lagoon, while most are 

 either brackish or fresh-water or land animals. Certain 

 marine species are practically land animals, since they 

 live where they cannot be immersed in water excepting 

 at extremely high tides, and even then they would not be 

 covered unless the waves dash upon the rocks where they 

 live. Adams 1 speaks of some species living in trees over- 

 hanging water. 



The shell of Neritina canalis Sowb. (No. 496), is cov- 

 ered by a thick, dark brown horny layer which, when 

 broken off, reveals a shell of peculiar whiteness. 



Pulmonifera. Many Gastropods that were originally 

 marine animals have doubtless become in the course of 

 many generations, and a longer or shorter period of 

 time, fresh-water or land animals. Most of these now 

 bear the name of Pulmonifera, since they no longer 

 breathe air dissolved in water, but are equipped with 



1 Genera of Moll., I, 1858, p. 381. 



