19 
THETIS * 
Gen. Char. An equivalved subequilateral bivalve ; 
more or less orbicular, and convex ; ligament 
marginal ; 3 or 4 small acuminated teeth 
about the hinge ; the line of attachment of the 
mantle ? has a deep sinus extending nearly to 
the beak; muscular impressions rounded, small, 
distant from the hinge. Ligament external* 
This genus bears some resemblance to Mactra; but the 
ligament is not internal, nor are there any remote teeth. 
It also comes near to Tellina, but its margin is not curved 
laterally. The shell is thin, with small incurved beaks : 
the leading feature is a line within, which, taking an irre- 
gular sweep from the anterior muscular impression over 
the middle of the valve, turns suddenly up, almost to the 
beak, then bends down again in a nearly parallel direc- 
tion for some distance, and at last proceeds to the pos- 
terior impression. This line has much the appearance of 
the mark'formed by the attachment of the mantle ; but 
the sipus in it is so remarkably directed that we are led 
to doubt, more especially as we have only observed fos- 
sil examples. There is no lunette or other external mark, 
the surface being nearly smooth and plain. The teeth 
about the hinge are unequal : the two interior ones are 
largest, conical, and slightly curved ; the other two, if 
there be more than one, are small. 
The fossils composing this genus have been referred 
to Venus, but are totally distinct. 
* The elegant symmetry in the form of the shells of this genus 
render it worthy of the name of this beautiful Sea Nymph. 
