OF CONCHOLOGY. 245 



shell and are soon worn, so as to present the appearance of six 

 white (lots radiating from the apex, and in all adult shells they 

 are wanting. This strongly reminds one of Scurria, which has 

 a white tip, sometimes marked with brown in a similar manner. 

 The peculiar waxen, or semi-translucent, brown outer layer, also 

 resembles the type of Scurria, which is also furnished with simi- 

 lar, radiating, fine strire, I have a thin, long, compressed speci- 

 men exactly intermediate between the typical form and A. pa- 

 leacea. 



ACM^A (?) INSTABILIS, Gld. 



Patella instabilis, Gld., Proc. B. S. N. Hist, ii, 150, 1846, 



Exp. Shells 9. 

 Nacella instabilis, Cpr., Sup. Rep. Br. As. 1863, p. 650. 



I have seen a large series of this species, but none with a per- 

 fect Mpex. It much resembles the last species, but appears to 

 be specifically distinct. The principal difference is in the sculp- 

 ture, which is in rather distant grooves instead of strise, and in 

 the size of the adult, which is much h'lrger than the last species. 

 The differences of form are probably due to a peculiar habitat. 

 The place of this species would seem, from the close resemblance 

 of the shell, to be probably in this division of the genus, but all 

 classification (other than provisional) from the shells alone is time 

 and work wasted. Neither this species nor the last, as far as we 

 know, have any relations with Naeella, as restricted. 



The species is rare at Sitka and Monterey and very common 

 at Vancouver. 



Subgenus Collisella *, Dall. 



Z/ottia, Acma'a, Tectura,, and Patella, sp. auct. 



Animal without any vestige of a branchial cordon. Muzzle- 

 frill simple, entire, not produced into lappets or tubercles. 

 Radula with the two inner laterals anterior, approximate, simple, 

 with long cusps ; next two posterior, with large, broad usually 

 simple cusps ; outer laterals minute, closely appressed and op- 

 posed to the cusps of the second pair ; pleurne pro^dded with one 

 or two very minute uncini close to the posterior corner of the 

 area. Formula 



2 or 1(2—1.1—2)1 or 2. 



This subgenus differs from Acma^a in the simple muzzle-frill, 

 the relative position and unequal size of the lateral teeth, and 

 in the very minute uncinus, which from its small size, is very 

 liable to be overlooked. The fact that the type of the genus 



*Frora Collis, a mound ; in allusion to tbeir shape. 

 17 



