80 MOLLUSCA FROM THE CRAG. 
and my two most extreme variations are here represented: all give indications of an 
opening for a byssus by an indentation or sinuosity on the anterior side of the 
ventral margin ; this character is imperfectly represented in the Mediterranean fossil 
by Philippi, nor is it well shown in that by Nyst, although the probabilities are they 
all belong to the same species: both of these authors speak of their shell as being by 
no means abundant. When perfect, it is ornamented with distinct radiating striz, 
the largest and most prominent at the two extremities; they are crossed by lines of 
growth somewhat irregular, and the surface is imperfectly cancellated. Upon the 
hinge line, on the anterior side, are three or four rather large and prominent teeth, 
which slope at an angle of about 45°, while those on the posterior, amounting to the same 
number and equally prominent, are nearly parallel to the hinge margin; in some 
small or young specimens, these teeth or prominences do not amount to more than 
two on each side, and all so arranged that when the valves are united, the one set 
interlock with those of the opposite valve. The place for the ligament is very small, 
and in perfect specimens there may be seen an entire row of crenulations just 
within the ventral margin of the shell; and a prominent ridge slopes down the interior 
from beneath the umbo, probably formed by the inner edge of the adductor muscle 
on the anterior side. In old specimens the mark of the mantle is deeply impressed, 
running parallel to the outer edge. 
Nucuta,* Lamarck, 1799. 
GuycIMERIs (sp.). Da Costa, 1778. 
TELLINA and Donax (sp.). Gmel. 
Arca (sp.). Linn. 
Poityoponta. Megerle, 1811. 
Generic Character. Shell equivalve, very inequilateral, ovato-trigonal, generally 
transverse, closed, nacreous, and in the recent state covered with an epidermis; often 
smooth, sometimes striated, or variously ornamented upon the exterior: hinge line 
more or less angular, furnished with a series of sharp, elevated, and angulated teeth, 
arranged on each side of the umbones, interrupted by a central and internal spoon- 
shaped projection, upon which is placed the ligament. Impression by the mantle 
without a sinus. 
Animal of the form of the shell, having the edges of its mantle plain dis- 
connected, and without siphonal tubes. Foot large, and capable of expanding into an 
ovate, pedunculated disc, with fimbriated edges, and by means of this organ it is 
capable of a considerable degree of locomotion, creeping like a Gasteropod at the 
bottom of the water. 
* Etym. Nucula, a little nut. 
