452 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.49. 



FamUy LYONSIIDAE. 



Genus LYONSIA Turton, 1822. 



Pandorina Scacchi, Osserv. Zool., p. 14, 1833; not of Bory St. Vincent, 1824, 



(Pohjgastrica.) 

 Magdala (Leach MS.) Brown, IUus. Conch. Grt. Britain, 1827, expl. pi. 11, figs. 



1, 2, 10. 

 Eiatella Brown, 1827, not Daudin, 1802. 



Myatella Brown, Illus. Conch. Grt. Britain, 2nd ed., p. Ill, 1844. 

 Osteodesma Deshayes, Encycl. M6th., vol. 3, p. 552, 1830, in table. Type — 



Mya norvegica Gmelin. 



LYONSIA STRIATA Montagu. 



Mya striata Montagu, Trans. Linn. Soc, vol. 11, p. 188, pi. 13, figs. 1, A, 1815 — 



Turton, Conch. Diet., p. 105, fig. 99, 1819. 

 Lyonsia striata Turton, Dithyra Brit., p. 35, pi. 3, figs. 6, 7, 1822. (Syn. ex 



parte exclus.) 

 ?Lyonsia bracteata Gould (as Osteodesma), Expedition shells, Proc. Boston Soc. 



N. Hist., vol. 3, p. 218, May, 1850; U. S. Expl. Expedition (Wilkes), p. 397, 



pi. 35, figs. 509, a, b, 1852. 



Distribution. — Shetland Islands; Arctic Sea; on the Pacific from 

 the Aleutian Islands south to the coast of Washington. Cat. No. 

 213722, U.S.N.M. 



On the northern coast of Europe and the British Isles occur two 

 forms of Lyonsia, one of which recurs on the Pacific coast of America. 

 One of these is the Mya norvegica of Gmelin,^ after Chemnitz, ^ and is 

 the well known Lyonsia norvegica of northern Europe. The other, 

 which was first named by Montagu Mya striata, differs by being a 

 thinner, smaller, and more slender shell, with the posterior dorsal | 

 margin nearly straight and more sharply truncate, while in norvegica 

 it is concavely arcuate; the test is almost translucent, the form less 

 inflated, the dorsal, and in many cases the basal edges near the margin, 

 conspicuously compressed, the umbones nearer the anterior end; and 

 the surface with usually a much greater profusion of adherent sand 

 grains than in the L. norvegica. The differences are fairly well indi- 

 cated by Turton's figures in his Conchological Dictionary (figs. 99 

 and 100, 1819), and, though there is more or less variation, yet no 

 one comparing adult and well preserved specimens can well avoid 

 noting the distinctions. 



The two have been generally regarded as mere mutations of the 

 norvegica type, but the presence of one and not of the other on the 

 Pacific coast would indicate a specific distinction. The fragments 

 remaining of Gould's type of hracteata indicate that it belongs with the 

 striata form, though his figure is not characteristic. Middendorff's 

 synonymy includes practically aU the northern species of Lyonsia 

 under one name, but his figures relate only to the following form. 



« Syst. Nat., vol. 7, p. 3222, 1792. 



«Conchyl. Cabinet, vol. 10, pi. 170, figs. 1647-8, 1788. 



