18 ACIVI/T5A. 



(2) Var. NACELLOiDES Dall (PI. 6, figs. 43, 44, 45), agrees with 

 A. instabilis in the blackish-brown color and in sculpture, but it is 

 less compressed laterally, and the basal margins are level, not 

 elevated at the ends. It is abundant, living on kelp, at Monterey. 



The proof of the alleged specific identity of instabilis with pelta 

 is incomplete. The specimens collected by Henry Hemphill and 

 described by him in Proc. Acad. Nat. Sci. Phila. 1881, p. 87, under 

 the name instabilis, are typical nacelloides. Hemphill found that 

 when these limpets live on the fronds and stems of kelp (Phyllospora) 

 they have always the iVoce//a-like form and are black or dark brown ; 

 when an individual leaves the kelp for a station on the rocks its 

 additional growth is of the normal black and white striped or tessel- 

 lated pattern usual in typical A. pelta. A specimen of this form, 

 beginning life as nacelloides and hecommg pelta, is figured on pi. 8, 

 iig. 95. 



A. INSTABILIS Gould. PI. 6, figs. 32, 33. 



Shell narrow and oblong, the basal margin elevated at the ends ; 

 texture thin ; slopes convex or bulging. Surface finely radiately 

 striated ; dark brown or black. Inside white or bluish, with or 

 without a faint brown spot in the cavity. 



Large specimens measure I2 inches (38 mill.) in length by I in. 

 (23 mill.) bi-eadth ; but the usual length is about one inch. 



Vancouve?' Id. to Monterey, Cal. 



P. instabilis Gld., Proc. Bost. Soc. N. H. ii, p. 150, 1846 ; U. S. 

 Expl. Exped. Atlas f. 454, 454a. — Nacella instabilis Cpr., et al. — 

 Acmcea instabilis Dall, Amer. Journ. Conch, vi, p. 245. 



This species has been considered by some to be a form of A. pelta ; 

 but no specimens connecting the two species have been reported, 

 although it is not at all improbable that such may occur. At present 

 A. instabilis has as valid grounds for being retained as a distinct 

 species as A. insessa or A. asmi. 



A. INSESSA Hinds. PI. 6, figs. 36, 37. 



Shell rather thin but strong ; outline oval or oblong, the sides 

 often parallel ; elevated, conical, the apex in front of the middle, 

 slopes convex ; surface smooth, jDolished. Color varying from yel- 

 lowish or olive-brown to chocolate ; inside usually very deep brown 

 with a lighter border. Apex blackish, sometimes having snowy dots 



