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divided centrally by a faint longitudinal furrow ; on eacli side of the ventral 
furrow are three or four broad radiating lateral folds, of which the last are 
faintly marked, and disappear almost completely towards the beak. The 
beak is small, slightly recurved, and not prominent. The hinge area is very 
small, and decidedly elliptical, not half as long as the transverse diameter of 
the valve. The ears are rounded. The dorsal valve, not quite so deep as the 
ventral valve, has a small well-defined ridge, slightly depressed in the middle. 
The lateral folds, three or four on each side, are usually more pronounced 
than in the ventral valve. The test is very thin. The external surface is 
ornamented by innumerable small granulations, rather regularly arranged 
quincunxially, and easily visible to the naked eye ; they are rounded near 
the beak, but slightly elongated elsewhere. Some concentric lines of growth 
are seen rather more numerous near the margin than on the rest of the shell. 
The internal structure is perfectly known, thanks to the many internal casts 
sent me by Mr. W. B. Clarke. The most perfect mould (PI, XI, Pig, 10) 
shows that the cardinal process is very large, that the muscular impressions 
are strongly marked, and as deeply hollowed in the shell as those of the 
preceding species. One specimen has preserved a part of its spiral sujoports, 
and shows that the lamellae were strong and hollow. 
Dimensions . — The well-preserved specimens are mostly small. One 
of them measures twenty-eight millimetres long, forty-four broad, and 
eighteen thick. The cast which I have figured as belonging to this species is 
ninety-three millimetres long, one hundred and eight broad, and forty-five 
thick. The muscular impression of the ventral valve is forty-six millimetres 
long, that of the opposite valve being rather shorter and indistinct. 
Relations and Differences . — I think that Mr, J. Morris has described 
and figured, under the name S. subradiatus, two perfectly distinct species ; 
one of these, of which he has given several figures on PI. 16 of Strzelecki’s 
work, seems to me identical with that described by W. Martin as 8. glaher, 
having no striking characteristic to distinguish it. Mr. Dana thinks 
this is the case with all the specimens figured by Mr. J. Morris, but on 
this point I do not agree with the learned American Naturalist. I think, 
on the contrary, that PI. 15, Pig. 5« of Strzelecki’s work, represents a very 
different species from the first ; as I have before stated, it is the only one 
that I consider is a young individual of S. Danvinii, J. Morris. It differs 
(') Mr. Morris agrees that this specimen has greater affinities with S. Danvinii. 
