182 MAZATLAN UNIVALVES 
This remarkable genus does not appear to be rich in species 
at Mazatlan. The magnificent S. gigas (? + characteristica), 
though traveling as far north as 8. W. Mexico, where it is not 
uncommon (P. P. C.), was entirely absent (adult) from this 
collection. 
239. Srpnonarra Lecanium, Phil. 
S. t. parva, plerumque ovatd, interdum subcirculari, ad cana- 
lem projiciente ; subconicdé, seu valde depress ; cinerea, fusca 
varve pictd ; epidermide tenut, adherente ; costis equaliter seu 
wmaqualiter distantibus, subacutis seu valde rotundatis, inter- 
stittis sepius costulis instructis ; costis majoribus Xi.-XXil. ; 
costis et costulis tenuissime striatis, striulis radiantibus, sub- 
rugulosis ; costis et margine interno interdum albidis ; vertice 
subcentrali, levi, planate ; pa gind ‘interné seu atraé, seu fused, 
rarius pallida, rarissime virescente ; margine seu irregulariter 
erenulato seu stellato ; costis rotundatis excurrentibus ; canali 
declhwi. 
Phil. in Zeit. f. Mal. 1846, p. 51, no. 18.—Menke in Zeit. f. 
Mole S47," 3p. Lit, 10% L. diagnosi aucta.—Nune diagnosi 
valde aucta. 
it cannot be expected that any one should recognize this 
species from so comprehensive a description; and yet any 
narrower definition would shut out shells that I am unable 
to separate from the typical forms. In its ordinary state the 
shell is subconiecal, thin, with a variable number of irregularly 
disposed, rather sharp ribs, somewhat projecting, and generally 
rubbed, shewing a white surface underneath. Between these 
ribs are an equally variable number of riblets, not projecting ; 
and over the whole surface, ribs included, are microscopic 
strie. When the creature grows in sheltered situations, it 
spreads out, becomes fiat and large, the ribs swell, become 
rounded, and develop at the margin into beautiful palmations. 
So Hiferent | is this form from the usual one that if they were 
each at all constant in their characters, no one would suspect 
them of bemg identical. Yet the variations in each are so 
extraordinary, and the intermediate forms so numerous, that 
I have found it impossible to separate them. Often, both in 
the flat and conical forms, the riblets develop into principal 
ribs, until all are nearly of the same size. It then closely 
approximates the next species. Interior of a rich iridescent 
brown or brownish black, generally light at the margin, some- 
