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 sti'ictly 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 Poll'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, 



