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 striae, 

 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. 



NUCULA,* Lamarck, 1799. 



GLYCIMERIS (sp.). Da Costa, 1778. 

 TELLINA and DON AX (sp.). Gmel. 

 ARC A (sp.). Linn. 

 POLYODONTA. 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, & little nut. 



