THE SHELL. 19 



peculiar structure ; and the Hippurite is distinguished by a 

 cancelLated texture, unlike an}^ other shell, except perhaps some 

 of the Cardiacese and Chamaceiae. 



Epidermis. All shells have an outer coat of animal matter 

 called the "epidermis" (or pe7nostracmn), sometimes thin and 

 transparent, at others thick and opaque. It is thick and olive- 

 colored in all fresh-water shells and in many Arctic sea-shells 

 (e. g. Cjin^ma and Astarte) ; the colors of the land-shells often 

 depend on it ; sometimes it is silky as in Helix sericea, or 

 fringed with hairs as in Trichotropis ; in the whelk and some 

 species of Triton and Conus it is thick and rough, like coarse 

 cloth, and in some Modiolas it is drawn out into long beard-like 

 filaments. 



In the cowry and other mollusks with large mantle lobes the 

 epidermis is more or less covered up by an additional layer of 

 shell deposited externally. 



The epidermis has life, but not sensation, like the human 

 scarf-skin ; and it protects the shell against the influence of the 

 weather and chemical agents ; it soon fades or is destroyed after 

 the death of the animal in situations where, whilst living, it 

 would have undergone no change. In the bivalves it is organi- 

 call}^ connected with the margin of the mantle. 



It is most developed in shells which frequent damp situations, 

 amongst decaying leaves, and in fresh-water shells. All fresh 

 waters are more or less saturated with carbonic acid gas, and in 

 limestone countries hold so much lime in solution as to deposit 

 it in the form of tufa on the mussels and other shells. But in 

 the absence of lime to neutralize the acid the water acts on the 

 shells, and would dissolve them entirely if it were not for their 

 protecting epidermis. As it is, we can often recognize fresh- 

 water shells by the erosion of those parts where the epidermis 

 was thinnest, namely, the points of the spiral shells and the 

 umbones of the bivalves, those being also the parts longest 

 exposed. Specimens of Melanopsis and Bith3mia become trun- 

 cated again and again in the course of their growth, until the 

 adults are sometimes only half the length they should be, and 

 the discoidal Planorbis sometimes becomes perforated by the 

 removal of its inner whorls ; in these cases the animal closes the 

 break in its shell with new layers. Some of the Unios thicken 

 their umbones enormously, and form a layer of animal matter 

 with each new layer of shell, so that the river action is arrested 

 at a succession of steps.* But the action of an acid is certainly 

 not the only cause of erosion ; it is assisted by the ravages of 

 various boring animals in some cases, and is frequently largely 



* On erosion of fluviatile shells, see Dr. James Lewis, Bost. Soc. Proc, 

 vi, 149. 



