244 NEW YORK STATE MUSEUM 



Limax maximus is found in gardens, where it is very destructive 

 to vegetation. During the day it secretes itself; but it may easily 

 be found, by the aid of a lantern, while feeding at night. 



POLYGYRA ALBOLABRIS Say, sp. 

 Shell 



The shell of Polygyra is turbinate, spiral; consisting in 

 P. a 1 b 1 a b r i s of five volutions, coiled around a hollow axis 

 known as the columella. The columella is open at the base in 

 immature specimens, but in adults it is closed by a growth of the 

 shell. When the shell emerges from the egg, it consists of one 

 volution only. Farther growth of the shell takes place in the 

 manner hereafter described. 



The shell consists of three parts or layers: the outer com- 

 monly, but wrongly, termed the epidermis — cuticle or Carpen- 

 ter's name, periostracum, is preferable; next a prismatic part; 

 finally a laminate part. The prismatic and laminate layers are 

 of about equal thickness, and together form the greater part of 

 the entire thickness of the shell. 



The cuticle is thin; in P. albolabris yellowish brown in 

 color. It is without sensation, like the human scarf skin. Its 

 office is to protect the shell from chemical action, which other- 

 wise would injure or destroy it. After the death of the animal 

 it fades and becomes brittle, or is entirely destroyed in situations 

 in which during life it would not be affected. 



The shell is formed by the thickened margin of the integument 

 covering the visceral mass. According to Carpenter, " the shell, 

 as before stated, is formed by the mantle of the shellfish, indeed, 

 each layer of it was once a portion of the mantle, either in the 

 form of a simple membrane or as a layer of cells, and each layer 

 was successively calcified (or hardened by carbonate of lime) and 

 thrown off by the mantle to unite with those previously formed."' 

 According to Huxley, the shell growth is not a case of conversion 

 but of excretion, and the shell is built up by successive excretions 

 of membranous laminae, in which granules of carbonate of lime 

 are deposited. 



