THE NAUTILUS. 43 



■come fainter on the last whorl, suture well impressed; aperture 

 rounded, broader than high, greatly indented above ; lip simple, 

 very little reflected below at its junction with the columella, very 

 sinuous above, its terminations joined by a very thin callus. 



Height f inch, breadth 1 inch. 



Habitat, Astoria, Oregon. 



In the strong rib-like sculpturing of the spire, depressed form and 

 sinuous lip, it resembles sportella. In its greater diameter, dark 

 greenish color, and the absence of the decussating sculpture on the 

 last whorl it approaches Vancouver ensis. 



All our American Selenites commence life with a finely granu- 

 lated shell. When they have attained about two whorls the striae 

 begins to appear and increase in strength as the shell increases in 

 size. 



It is well known that all shell-bearing mollusks construct their 

 shells, in obedience to the laws of their constitutional characteris- 

 tics and the environment, among which I include affinity of matter 

 and mechanical skill, the later a faculty possessed to a greater or 

 less degree by all animals. Some individuals in a colony of shells 

 display greater mechanical skill tluin others, or possess stronger im- 

 itative powers, and closely follow the lines and styles of their fore- 

 fathers, strictly attending to the details of sculpturing, not omitting 

 a rib or line. Other individuals of the same colony, not having this 

 imitative faculty so strongly developed, may change or vary the 

 form of the shell by constructing it with more convex whorls gener- 

 ally resulting in a narrower or more elevated shell ; or they may 

 flatten the whorls, resulting in a broader and depressed form. Some 

 modification of the umbilicus generally follows the change in the 

 form of the shell. In both cases the sculpturing may be what we 

 call characteristic of the species, or may be more or less modified, 

 by the omission of one, two or more ribs, or the ribs may be more 

 irregular in shape. A few lines may also be dropped, perhaps some 

 added, or the entire surface may be modified in obedience to the laws 

 of the mechanical skill possessed by the individual, and the afiinity 

 of matter secreted by the animal for the purpose of constructing the 

 shell. An exaniination of a large number of Selenites concava and 

 of our west coast forms, convinces one that the entire group of 

 American Selenites is the offspring of a single common type. 



