PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 335 



(Fisher!) to Puget Sound, W. T, (Swau and Keuuerly!). Abundant 

 fi'om low water to eighty fathoms on stones ^ud. shells, sometimes attain- 

 ing the length of an inch, but usually about four-tenths of an inch long. 

 Five hundred and twenty-seven specimens examined. 



This is the largest and most abundant species of the family. In it, 

 beside differences in dentitiou, the apex is simply i)ointed or blunt, not 

 deciduous, as in the typical Lepeta. The sculptiuc is usually faint, but 

 sometimes raised in beautiful concentric frills, from w^hicli the name was 

 derived. Small specimens from slight examination have been quoted as 

 L. caeca by authors. It has not yet been found north of the Aleutians. 



C. concentrica var. instabilis. 



f Cryptohranclda instuhili« DiiW, 1. c. p. 145, pi. 15,1'. ti. 



I am now convinced that the provisional name which I applied to this 

 singular form is only of varietal value. It seems, from later specimens, 

 to be a form which, fiom living on the stalk of Nereocystis, has become 

 peculiarly arcuated and greatly thickened, much like Acincea instabilis, 

 which has the same habit. It has only been found at Sitka in small 

 numbers, dead, in 10-15 fathoms. 



Cryptobranchia alba. 



a alba Dall, 1. c. p. 145, pi. 15, f. 3 a-d, 1869. 



Rab. — Plover Bay, E. Sib., Dall! Seniavine Straits, Stimpson! 

 Akutan Pass, Aleutian Islands, Dall ! Dead on beach. Alive at six- 

 teen fathoms, gravel. Twenty-four specimens examined. 



This species appears to fill the gap between the distribution of L. 

 cceca and C. concentrica. It is easily distinguished from the latter by its 

 smooth surface and rounded apex and back, beside anatomical charac- 

 ters. It rarely reaches nearly an inch in length, and is of the purest 

 whiteness. 



Extra-limital Species. 



Subgenus Pilidium Forbes. 



Pilidium Forbes, Athenaeum, Oct. 0, 1849, p. 1018. — Forbes &. Hanley, Brit. Moll, ii, 

 p. 440, 1849; not of Middendorf, Sib. Eeise, p. 214, 1851.— Dall, 1. c. 1869 

 (synonymy, etc., in full). 



lothia Gray, not Forbes, 1854 (cf. Dall, 1. c. 1869). 



Tectura Jeffreys, 1865, not of Gray (1847), nor of authors. 



Scutellina Chenu (pars), Sars, not of Gray, 1847. 



Pilidium fulvum. 



rateUafulva O. F. Muller, Prodi". Zool, Dan. p. 227, 1776. 



Pilidium fulvum Forbes, Athenaeum, 1. c. Oct. 6, 1849. — Dall, 1. c. 1869. 



Pilidiuvi rubellum Stm. Checklist Sh. N. Am. E, Coast, No. 312, 1865. 



Tectura fulva J eSreys, Br. Couch, iii, p. 250, 1865. 



Patella foi-bes a J. Smith, Wern. Soc. Mom. viii, p. 107, pi. ii, f. 3. 



Scutellina fulva G. O. Sars, Moll, Keg. Arct. Norv. p. 122, 1878. 



Rah. — ^Northern and Arctic seas of Eastern America and Europe; 

 doubtfully reported from the Adriatic, Avhere, if it be correctly identified, 



