BIVALVIA. 87 



however, sufficient diiFerence to remain as specimens of a distinct variety, should they 

 be hereafter degraded from their isolated position. I have seen this species from the 

 Coralline Crag only, where N. nucleus is found in abundance, and of all sizes, but 

 none of which seem to form any very close connection with this shell. British 

 Conchologists retain as distinct in this genus forms with less variation of character 

 than what is exhibited in our fossil specimens. 



Nucula nitida. Sow., appears to offer less pretensions for specific distinction, than are 

 presented by the angulated outline of our fossil, but probably the whole three will 

 merge into one species. 



Nucula proxima of American authors, according to figures and descriptions, much 

 resembles our fossil, and is probably only another specific variation of a shell with 

 a very extended range. 



Nucula radiata, Hanley, differs nothing in form from some of our Crag specimens, 

 and if it be a distinct species, may also have been in existence during that early 

 period. 



Leda,* Schumacher. 1817. 



Nucula (sp.). Lam., 1801. 



Arca (sp.). Mont., 1803. 



Lembulus. Leach, MS. 1819. 



Lembula (sp.). llisso, 1826. 



Dacromya. Agass., 1839. 



Yoldia. Moller, 1842. Lovin, 1846. 



Leda. Lovin, 1846. 



Moldia. Gray, 1847 (misprint). 



Generic Character. Shell equivalve, inequilateral, elliptical or fig-shaped, posteriorly 

 more or less angulated or acuminated ; smooth or transversely striated, covered by 

 an epidermis in the recent state ; umbones small, approximate. Hinge furnished with 

 numerous teeth arranged in a linear series, curved or slightly angular, interrupted in 

 the centre, or immediately beneath the beaks by a triangular fossette for the reception 

 of the ligament ; impressions by the muscles ovate or subangular, that by the mantle 

 more or less sinuated. 



Animal transversely ovate, mantle open in front with simple, sometimes fimbriated, 

 margins ; foot large and discoidal, with serrated edges ; posterior side of the mantle 

 prolonged into two, partially united, slender and unequal siphonal tubes. 



The genus Nucula, as established by Lamarck, was intended to contain all those 

 shells which had a hinge furnished with a line of sharp prominent teeth, separated 

 at the umbo by an angular depression for the reception of the ligament, and placed 



* Etym., a propername. 



