BIVALVIA. 61 



ligament, but they extend along the whole line of hinge or dorsal area ; the posterior 

 side in some specimens is much rounded, so as to give a cylindrical form to the shell, 

 while in others there is an angular slope on the posterior side, from a little beyond 

 the termination of the hinge line to the ventral margin, which gives a greater width 

 to that part, thereby producing a different form, and which, in consequence, were that 

 a permanent character, might be considered a distinct species, but my specimens are 

 exceedingly variable, so as to present no marked distinction between the two. 



5. MODIOLA SERICEA, Bronn. Tab. VIII, fig. 3. 



MODIOLA SERICEA. Bronn. Ital. Tert. Geb., p. 122, No. 649, 1831. 



Phil. En. MoU. Sic., vol. i, p. 71, t. 5, fig. 14, 1836. 



Id. - vol. ii, p. 52, 1844. 



Nyst. Add. a la Faime. Conch., p. 444, No. 38, 1842. 

 HYALINA. S. Wood. Catalogue, 1840. 

 MYTILUS SEKICEUS. Gold/. Pet., vol. ii, p. 179, pi. 131, fig. 12, a c, 1841. 



Nyst. Conch. Foss. de Belg., p. 271, pi. 21, fig. 2, b e, 1844. 



JE. Sism. Synop. Meth. An. Invert., p. 15, 1847. 



Bronn. Ergeb. meiner. naturh.-6'kon Reise, vol. ii, p. 620, sec. Nyst. 



Spec. Char. Testa tenuissime, subhyalind, elliptico-ovatd, tumidd ; valde incequilaterd ; 

 striatd, striolis exilibus confertis ; umbonibus prominentibus recurvis ; maryine ventrali 

 subrecto, tenuissime crenulato. 



Shell thin delicate, subhyaline, of an elliptical or ovate form, tumid ; very inequi- 

 lateral ; covered externally with numerous close-set, extremely fine, radiating striae ; 

 umbo projecting beyond the anterior side ; ventral margin nearly straight and 

 finely crenulated. 



Locality. Cor. Crag, Ramsholt and Sutton. 



It is very difficult to obtain specimens of this species in perfection, on account of 

 its extreme thinness, although they appeared at Ramsholt to be by no means rare, but 

 they are generally more or less broken or distorted. 



This beautiful species is of an ovate or elliptical form, very tumid, excessively 

 thin and semitransparent ; it is covered over its whole surface with extremely 

 fine radiating striae, crossed occasionally by irregular lines of growth, but not 

 regularly decussated ; the ligamental area extends about half the distance of the 

 dorsal portion, its termination forming a very obtuse angle ; the umbo is terminal and 

 somewhat prominent, curved, and slightly projecting beyond the anterior margin. 



This appears at present to be known only as a fossil. My largest specimen 

 measures an inch and a quarter in its longest diameter. 



