442 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.49. 



The Pacific coast species have mostly been confused together, their 

 supposed variability is largely due to confounding different species 

 under one name. The nestling species, hke T. curia, however, are 

 naturally modified by their situs. 



Genus THRACIA (Leach MS.) Blainville, 1824. 



Type. — T. corbuloidea Deshayes. 



(For full synonymy and discussion, see Trans. Wagner Free Inst, of Science, vol. 3, 

 pt. 6, p. 1522, 1903.) 



THRACIA CURTA Conrad. 



Thracia carta Conrad, Journ. Acad. Nat. Sci. Phila., vol. 7, p. 248, pi. 19, fig. 8, 

 1837. Type locality, Santa Barbara, California. 



The shell, when normal, has much the shape of T. corbuloidea 

 Deshayes, or T. conradi Couthouy, but never grows as large as the 

 latter. It is frequently found nesthng in rock cavities and borings, 

 when young, and in such cases its general aspect conforms to the 

 walls of the cavity, and so we have specimens simulating the profile 

 of Petricola, Lithophaga, or a pholad. One such was that Conrad 

 originally figured. The large series in the United States National 

 Museum enables me to connect the various mutations with the normal 

 form. This species is chalky white with a dehiscent brownish perio- 

 stracum, and is densely coarsely granulose. A well grown specimen 

 measures: Length, 53; height, 42; diameter, 22 mm.; with the ver- 

 .tical from the beaks falling 26 mm. in front of the posterior end. A 

 larger fragment when complete must have measm-ed over 60 mm. in 

 length. I have not found a lithodesma, which perhaps is present in 

 the young. The pallial sinus is linguiform and nearly reaches the ver- 

 tical from the beaks. Cat. No. 74216a, U.S.N.M. 



This species ranges from Icy Cape, Arctic Ocean, to Bering Strait, 

 Plover Bay, the greater part of Bering Sea and the Aleutians, and 

 southeast to San Diego, California. The more southern specimens 

 do not attain the size of those from Alaska, judging by those which 

 have been collected. The fossil T. trapezoidea Conrad is a more elon- 

 gated shell. 



THRACIA BERINGI, new species. 



Thrada heringi (Dall) J. G. Cooper, Cat. sheila in State Mining Bureau, 1894. 

 (Name and distribution only. List not paginated.) 



Shell of moderate size, white, with a dehiscent yellowish perio- 

 stracum (usually lost), and looking much like a Macoma sahulosa. 

 Valves nearly equal, nearly equilateral, with low beaks and a con- 

 spicuous external ligament enfolding the resilium. Surface with in- 

 cremental irregularities and in the periostracum numerous small 

 wrinkles, but without perceptible granulation. Pallial sinus large, 

 rounded, not reaching the vertical from the beaks. Valves rounded 

 at both ends, the posterior dorsal area inconspicuous, not bounded 



