hinge central, with a single tooth in one valve (soine- 
limes a second rudimentary one) and two in the other, 
behind which the margin is callous. 1...2. Rare. 
S. Deciivis.—Turt. Con. Die. p.164. t. 22. f. 80. 
—F. p. 460.—JList. t. 321.—Oblong, rounded at the 
extremities, dorsal margin slightly convex, ventral 
straight; thin, semi-transparent, with a dark brown 
cuticle, irregularly and concentrically striated, beaks 
subcentral; sloping gradually to the extremities; 
slightly indented before the beaks; teeth strong, two 
in each valve, one of them concave and in one valve 
one is oblique, behind which the margin is callous. 
$...2. Scilly Isles. 
S. Fraeiuis. Pult. Dor. p. 28.—Mont. p. 51.— 
F. p. 460.—Psammosia Te@niata. Turt. B. pp. 85.— 
Oblong, rounded at the extremities, a little contracted 
in the middle, dorsal and ventral edges subparallel, 
thin, transparent, smooth, with a greenish cuticle, some- 
times marked with a longitudinal reddish stripe from the 
hinge towards the ventral margin; beaks subcentral ; 
in one valve two erect teeth, one of which is pointed the 
other with one subulate tooth: an internal longitudinal 
TIDs 4 30000,95. 
