THE HORN-MOUTH. 27 



and may be supposed to be of the average natural 

 size, unless something is stated to the contrary. 



The shell of this species is about two inches long, 

 of a dingy white color, somewhat marked w^ith brown, 

 and each whorl usually has three distinct varices 

 with rounded knobs between them. 



Cei^ostoma foliatiim, Gmel., fo-li-a^-tum, which is 

 thought by some to be but a variety of the last species, 

 has a larger shell, which is ver}^ conspicuously marked 

 by three broad, wing-like varices, which appear to be 

 made up of overlapping plates, like shingles on the 

 roof of a house. I have only one specimen in my 

 collection, but that one is a beauty. It was found alive, 

 under a stone, and was given to me a few minutes 

 after its capture. 



Whether we shall consider C. foliaticm as distinct 

 from the species shown in Fig. 8, or whether it is but 

 a mere variety of the same species, is a matter of very 

 little consequence in itself; but it is an inquiry which 

 opens the door to very intricate questions concerning 

 the definition of the term species. 



By some it is believed that a species includes all 

 the descendants from an original pair of ancestors. 

 Dr. Isaac Lea, that patriarchal naturalist, said : " A 

 species must be considered a primary established law, 

 stamped with a persistent form — a type, pertaining 

 solely to itself, with the powder of successively repro- 

 ducing the same form and none other." 



Others, who have studied and observed extensively, 

 believe that species and genera do not exist in nature, 

 but are merely convenient terms to designate groups 

 of similar forms and characteristics ; and that, like 

 other provisional terms, they will pass away when a 

 better system of naming is devised. 



