OF CONCHOLOGY. 245 



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

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

 are wanting. This strongly reminds one of S'curn'a, which has 

 a white ti'p, sometimes marked with brown in a similar manner. 

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

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

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

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

 leacea. 



ACM.EA (?) INSTABILIS, Gld. 



Patella insfabilis, 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 apex. It much resembles the last species, but ap'pears to 

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

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

 the size of the adult, which is much larger 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 provisioniil) from the shells alone is time 

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

 know, have any relations with Nacella, as restricted. 



The species is rare at Sitka and Monterey and very common 

 at Vancouver. 



Subgenus Collisblla *, Dall. 



Lottia, 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 ; pleurie provided 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 Acmcea 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 genua 



*Froin Collis, a mound ; in allusion to their shape. 

 17 ■ 



