10 A MANUAL OP AMERICAN LAND SHELLS. 



brane investiDg the whole, the interior surface of which is usually stud- 

 ded with numerous rhombic, microscopic crystals of carbonate of lime, 

 some species, however, having a hard enveloping calcareous shell, of the 

 consistence of that of a bird's egg ; of an inner thin, transparent, shin- 

 ing membrane which immediately incloses a transparent and somewhat 

 viscid fluid, analogous to the albumen of birds' eggs ; of the albumen 

 itself, and of the vitellus, which, possessing the same degree of trans- 

 parency as the albumen, cannot be distinguished from it at this time. 

 The elastic eggs when first laid are often flaccid, and seemingly only 

 half full of fluid, but they soon absorb moisture and become distended. 

 The embryo animal, with its shell, is observable in the albuminous fluid 

 in a few days after the egg is laid. Its exclusion takes place, under 

 ordinary circumstances, in from twenty to thirty days, according to the 

 state of the atmosphere. Warmth and humidity hasten the process, 

 while cold and dryness retard it to an almost indefinite extent. The 

 hatching of eggs laid late in the autumn is often interrupted by the ap- 

 proach of cold weather and of snow, and delayed until the next spring. 



The young animal gnaws its way out of the egg, and makes its first 

 repast, of the shell which it has just left. It consists at first of about 

 one and a half whorls, the umbilicus being minute, but open. Its 

 growth is rapid, and it has usually increased in magnitude three or 

 four times before the close of the first year. 



In the month of October, or at the epoch of the first frost, the snail 

 ceases to feed, becomes inactive, and fixes itself to the under surface 

 of the substance by which it is sheltered, or partially burrows in the 

 soil, and with the aperture of the shell upward, disposes itself for its 

 annual sleep or hibernation. Withdrawing into the shell, it forms over 

 the aperture a membranous covering, consisting of a thin, semi-trans- 

 parent mixture of lime, mucus or gelatine, secreted from the collar of 

 the animal. This membrane is called the epiphragm. It is formed in 

 this manner : The animal being withdrawn into the shell, the collar is 

 brought to a level with the aperture, and a quantity of mucus is poured 

 out from it and covers it. A small quantity of air is then emitted from 

 the respiratory foramen, which detaches the mucus from the surface of 

 the collar, and projects it in a convex form, like a bubble. At the same 

 moment, the animal retreats farther into the shell, leaving a vacuum 

 between itself and the membrane, which is consequently pressed back 

 by the external air to a level with the aperture, or even farther, so as 

 to form a concave surface, where, having become desiccated and hard, 



