THK NAUTILUS. 103 



It is quite evident that the opercula vary in substance and form ; 

 that these variations are related to groups and genera ; that in some 

 instances (Opisthoporus, Stoastoma) they present in one family a 

 unique and generally well-maintained uniformity of structure. But 

 their determinative character has been overestimated. The promi- 

 nent characters of the opercula are repeated in diverse sections of 

 the entire group, and the elements of their composition are more 

 nearly significant of habitat than they are of systematic differences 

 in the shells themselves. 



It is noteworthy in this connection to recall the demonstration 

 made by Blanford (Annals & Mag. Nat. Hist., 1869), that the oper- 

 cula of Georissa (Blanford's genus) and Hydrocena Parreyss are 

 almost identical, whereas the former shell is entirely terrestrial, and 

 the latter marine. 



(A note of importunity to traveling or exploring naturalists may 

 be appropriately inserted here. The character of the environment 

 of species or genera of land shells is generally not closely enough 

 observed. The moisture, temperature, mineral nature of the ground, 

 seasonal features, and frequency and amount of sunlight, the vege- 

 tation, are all. influential factors in the physiological life of shells. 

 They do, in land shells especially, have a clearly marked relation to 

 the coloring, thickness and operculum of the shells, and they are not 

 as particularly dwelt upon by collectors as they might be, both for 

 the purpose of elucidating the effect of environment and the some- 

 times misleading rules of systematists.) 



Poey has called attention (Memorias sobre la Historic, Natural de 

 la hla de Cuba] to a striking illustration of local influence in the 

 sierras of the Isle of Pines. The Sierras de Casas and de Caballos 

 are respectively a short distance to the west and east of Nueva- 

 Gerona ; in the latter region the shells display regular spires, and 

 the crustaceans are destitute of spinose surface, as Trochatella stellata, 

 Pineria beathiana, Cyclostoma pupoides, among the shells ; Oniscus 

 sp.? among the crustaceans. In the former area the Trochatella is 

 represented by a denticulate, roughened, and loosely coiled shell 

 (constellata); the Pineria resembles a screw (terebra); the Cyclostoma 

 separates its last volution (moreletianum), and the Oniscus presents 

 spiny asperities, preserving otherwise a very similar appearance to 

 the de Casas species. 



The geological formations are identical, and Poey has suggested 



