BIVALVIA. 15 
of irregularities produced by its often confined position, the edges of the lower valve 
of this variety have sometimes a fimbriated character, like what has been called 
denticulata, parts of the larger valve projecting considerably beyond the upper, more 
especially on each side of the hinge, where the shelly matter is pushed up or elevated, 
so as to have, in some specimens, the fanciful resemblance to a spectral appearance 
produced by a person with extended arms beneath a cloth, which suggested the name 
to the late Rev. G. R. Leathes. The more common variety of the present day, and the 
one by which our markets are supplied, does not appear in this deposit, at least, there 
is no specimen in my cabinet strictly resembling that shell, although there is no doubt 
the specimens figured are mere modifications of the same species. 
A detailed description of this common and well-known shell is unnecessary, as the 
form and appearance of almost every specimen will present some diversity. It may, 
however, be observed, that in the interior of the upper valve of some specimens of 
the thick variety, a little within the hinge, is a small indentation or puncture not 
present in all, and may be often seen in the common variety of the recent shell: for 
what purpose this is intended or how produced Malacologists have not informed us, as 
it appears to have been overlooked, probably, as of no importance ; it certainly is of no 
use as a specific determination, as the same may be seen in specimens of a very dif- 
ferent species from the deposits of the older Tertiaries. The form of the impression pro- 
duced by the adhesion of the adductor muscle, it is to be feared, is a character of no 
great dependence, assuming, as it often does, a variation in shape conforming, in some 
slight degree, to the outward form or contour of the shell: in the thick variety, this is 
generally more or less ovate, its longer axis being from the anterior to the posterior 
side, and slightly contracted in the middle of the upper part, ascending and some- 
what pointed towards the posterior; rounded on the lower side, and rather broader 
on the anterior, or towards the middle of the shell; in the var. spectrum or para- 
sitica, this mark is as broad as it is long, and of nearly an orbicular or roundedly 
quadrate form. 
A long and interesting history of this species, and of its commercial value, is given 
by the authors of the ‘Hist. of Brit. Moll.,) who consider the English coast as its 
peculiar province, and although it has a very considerable geographical range, it is 
nowhere obtained in such great perfection as in our own seas; there is not in 
that work any notice of this shell, as an inhabitant of the Mediterranean Sea, and it is 
certainly not given as a living species by Philippi in his ‘En. Moll. Sic., nor by 
Payraudeau in his ‘Catalogue of Corsican Shells ;’ but in Poli’s splendid work ‘ Test. 
utrius. Sicil. 1795,’ is figured and described a group of shells, as well as the animal, of 
what appears to belong to this species, and from the general accuracy of that obser- 
vant author, there is very little doubt the specimens were procured in those seas. 
From the List of Synonymes, it will be seen, that several authors have given this as a 
fossil from the newer Tertiary formations of that part of the world, where it has been 
considered by many to be no longer in existence; and the fossil from the Morea, 
