BIVALVIA. 107 
NUCULA. Lamarck. 
Generic Character. Shell ovately trigonal or nut-shaped, smooth, or occasionally 
sculptured ; nacreous, inside iridescent ; siphonal region short or truncated ; the umboues 
never prominent; hinge with a row of more or less numerous angular and elevated inter- 
locking teeth ; connexus cartilaginous ; palleal line simple. 
Animal of the shape of the shell; margins of the mantle disconnected all round ; foot 
large, capable of being expanded into a disc, and ornamented with fimbriated edges. 
The peculiarity of this genus consists in having the larger portion of the shell on the 
pedal side, and the umbo pointing in the opposite direction, an arrangement contrary to 
that which prevails in the generality of bivalves, and also in having the spoon-shaped projec- 
tion within the hinge-margin, on which is placed the cartilaginous connector on the pedal side 
of the umbo. The hinge-line forms nearly a right angle, but this diverges into an obtuse 
one in the aberrant species, where an extension of the shell, on the verge of the genus, 
approaches the ovate or elongated form of Leda. 
The animal of this genus, the inhabitant of the shell which is the type, is said not to 
have any siphons, and that the margins of the mantle are disconnected. In the approxi- 
mating genus Leda, the mantle in the siphonal region is connected so as to form two 
distinct tubes, which are capable of considerable exsertile extension. ‘The animal of Mucula 
proper has the mantle open all round; but in those species which have an extension on 
the siphonal side approaching Zeda, it will probably be found that the margins of the 
mantle in the siphonal region are partly connected, so as to separate the incoming from 
the outgoing current. 
In this genus the greater number of the fossil species have the interior or ventral 
margins of the shells ornamented with crenulations. ‘These crenulations are found in those 
species only in which the exterior of the shell is covered with radiating lines ; they are 
apparently due to the fimbriated edges of the mantle, and do not extend to the edges 
of the dorsal margins, even where the area of dentition is limited. The ventral margins of 
the mantle in JV. zucleus are said to be plain; but I imagine they must be very finely fim- 
briated, in order to deposit the elevated layers of shelly matter which produce the radia- 
tions. These rays are most conspicuous upon the under surface, which is sometimes covered 
over with a coating of enamel-like material, so as to obliterate or at least to obscure the rays ; 
but when the margins are crenulated, I presume they will always be more or less visible. 
The shells of the species which have the margins smooth will be entirely free from 
radiating striee, and the edges of the mantle of those animals are probably quite plain. 
The shells in the living state are covered with an epidermis, remains of which may be 
occasionally observed upon specimens of the Eocene deposits. Some have their radiations 
strongly decussated by elevated lines of growth ; a few species also have a peculiar ornament 
in a zigzag form, and for these a sub-genus has been proposed, under the’ name dei/a, 
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