BIVALVIA. 87 
however, sufficient difference 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 JV. 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 provima 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. 
Lepa,* Schumacher. 1817. 
Nucvta (sp.). Lam., 1801. 
Arca (sp.). Mont., 1803. 
Lemputus. Leach, MS. 1819. 
LemBvLa (sp.). Risso, 1826. 
Dacromya. Agass., 1839. 
Youpra. Miller, 1842. Lovén, 1846. 
Lepa. Lovén, 1846. 
Motpra. 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 Wucula, 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 proper name. 
