TROPIOPODA. LAMELLIBRANCHIATA. 301 



1. Sphenia Swainsoni. Swainsori's Sphenia. 



Shell oblong, with the anterior end rounded and closed, the 

 posterior longer and gaping, the umbones rather prominent, 

 both valves considerably convex, thin, concentrically rugoso- 

 striate ; the laminiform tooth of the right valve oblongo-angu- 

 late, narrower behind; the concave tooth of the left curved 

 downwards and outwards beneath the umbo ; the colour white, 

 the thin epidermis yellowish-grey ; the inside bluish-white, 

 glossy behind, anteriorly dull. Length about half an inch, 

 height half the length. 



It varies considerably in form, being oblong, oval, or sub- 

 cuneate. 



Common off the coast of Aberdeen, among the roots of algae 

 and corallines. First observed by me, in August, 1842, 

 among shells cast on the sands near the mouth of the Dee, 

 and in September, at Don-mouth alive. Also among shells 

 sent by Mr. Alexander Murray from Cmden. After storms, 

 it is sometimes very abundant on the sands. Some suppose it 

 to be the young of Mya arenaria. 



Sphenia Swainsoni. Turt. Brit. Biv. 37. PI. 19. f. 2. Sphenia 

 Swainsonii. Flem. Brit. Anim. 466. Mya Swaiusom. Forbes, 

 Malac. Mon. 54. 



2. Sphenia costuldta. Ribbed Sphenia. 



Shell ovato-elliptical, both ends being equally rounded, the 

 anterior much shorter, the umbones small and slightly promi- 

 nent ; the valves convex, very thin, semitransparent, glossy, 

 concentrically striulate, and with (about) fifteen radiating little 

 elevated ribs ; the tooth in the form of an elongated thin la- 

 mella of small extent; the colour hyaline-white. Length a 

 twelfth and a-half, height one-twelfth. 



This most delicate and beautiful shell is decidedly a Sphenia, 

 and obviously very different from any of the smaller species 

 of other genera with which it might, from its form, be con- 

 founded. I have seen only a single valve, picked by Miss 

 Macgillivray from among shell sand, from the Bay of Cruden, 

 sent by Mr. Murray, in November, 1842. 



GENUS 4. MONTACTJTA. 



Shell ovate or oblong, equivalve, inequilateral, closed, 

 or very slightly gaping, very thin, glossy, concentrically 

 striulate. Umbones very small, little prominent. Hinge 

 2 c 



