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 S. W. Mexico, where it is not 

 uncommon (P. P. C), was entirely absent (adult) from this 

 collection. 



239. SiFHONAEiA Lecanium, Phil. 



8. t. parva, plerumqtie ovatd, interdum subcirculari, ad cana- 

 lem projiciente ; suhconica, sen valde depressd ; cinered, fused 

 varie pictd ; epidermide tenia, adhoerente ; costis (equaliter seu 

 inoequaliter distantibus, subacutis seu valde rotundatis, inter- 

 stitiis scepitis costulis instructis ; costis majoribus xii. -xxii. ; 

 costis et costulis tenuissime striatis, striulis fadiantibus, sub- 

 rugulosis ; costis et margine interno interdum albidis ; vertice 

 subcentrali, Icevi, planatd ; pagind \internd seu atrd, seu fused, 

 rarius palUdd, rarissime virescente ; margine seu irregulariter 

 crenulato seu stellato ; costis rotundatis excurrentibus ; canali 

 declivi. 



Phil, in Zeit. f. Mai. 1846, p. 51, no. \%.—Menke in Zeit.f. 

 Mai. 1847, p. 177, no. 1. diagnosi aucta. — Nunc 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 subcorneal, thin, with a variable nimiber of ii-regularly 

 disposed, rather sharp ribs, somewhat projecting, and generally 

 rubbed, shewing a white surface underneath. Between these 

 ribs are an equally variable niunber of riblets, not projecting ; 

 and over the whole surface, ribs included, are microscopic 

 stria?. When the creature grows in sheltered situations, it 

 spreads out, becomes flat and large, the ribs swell, become 

 rounded, and develop at the margin into beautiful palmations. 

 So different is this form from the usual one that if they were 

 each at all constant in their characters, no one would suspect 

 them of being 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 a!id conical forms, the riblets develop into principal 

 ribs, until all arc 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 margm, some- 



