300 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



1. Lyonsia Norwegka. Pearly Lyonsia. 



Shell oblong, much compressed, with the valves very thin 

 and brittle ; the umbones small ; the anterior end shorter and 

 rounded, the posterior truncate ; the dorsal line nearly straight, 

 the ventral margin arcuate ; the surface white, with concentric 

 obsolete stria?, becoming distinct and irregular at the posterior 

 end, and numerous divergent stria?, the slender ridges between 

 which are more prominent and nodulose in .the space between 

 the dorsal line and an inconspicuous ridge or fold from the 

 umbo to the lower angle of the truncation ; the epidermis pale 

 greyish-yellow ; the inside pearly and iridescent. Length 

 about an inch and a-half, height about ten-twelfths of an inch. 



A portion of a valve found by Mr. Davidson, brought from 

 deep water off Aberdeen, in the Spring of 1 842. 



Dr. Fleming makes this shell a My a; but I think its characters 

 are quite prominent enough to render it generically distinct. 



Mya Norwegica. Gmel. Syst. Nat. 3222. — Mya striata. Mont. 

 Linn. Trans, xi. 188. PI. 13. f. 1. A.— Mya pellucida. Brown, 

 Werm. ii. 105. PI. 24. f. 1.— Lyonsia striata. Tart. Brit. Biv. 34. 

 PI. 3. f. 6, 7. — Mya Norwegica. Fleni. Brit. Aniin. 463. 



Genus 3. Sphenia. 



Shell oblong, inequivalve, anisomeral, gaping behind. 

 Hinge of the left valve with a thin dilated horizontal 

 plate-like tooth, anteriorly abrupt, sloping backwards 

 and adnate ; that of the left valve with a concave tooth, 

 and an anterior minute prominence ; ligament internal ; 

 umbones small, pointed, curved a little forwards, nearer 

 the anterior end ; muscular impressions moderate ; si- 

 phonal impression very deep, broad, and obtuse. 



Animal oblong, compressed ; with the mantle closed, 

 leaving only a rather small opening for the foot, and 

 forming behind a compressed very extensile tube, con- 

 taining two siphons, which are united to the end ; foot 

 very small, oblong, compressed, obtuse. 



This genus, instituted by Turton, is closely allied to 

 Mya, from which it in fact does not differ very essen- 

 tially. The species occur imbedded in rocks, and other 

 hard substances, among corallines and the roots of fuci, 

 as well as in mud and sand. 



