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 umbones 

 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 Nucula 

 proper has the mantle open all round; but in those species which have an extension on 

 the siphonal side approaching Leda, 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 N. nucleus 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 striae, 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 Jcila, 



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