284 MOLLUSCA FROM THE CRAG. 
a considerable variation in that character may be observed. Great reliance has been 
placed upon the size of the gape for the foot on the anterior side, but I believe such 
distinction to be of very little value, as in some of my specimens it is nearly closed, 
while in others it is widely open, and with all intermediate magnitudes. The variety 
Ipsviciensis, found at Ramsholt, appears to differ most from the Sicilian shell, and may 
be considered its extreme range in variation, being more equilateral and straight ; but 
some of my specimens from near Orford, have nearly the same inequilaterality as the 
Sicilian fossil, with the like obliquity. In P. Norvegica, a difference exists between the 
comparative magnitudes of the two sides of the recent, as well as between the Sicilian 
fossil, and my specimens from Chillesford, equally evident with what may be seen in 
this species ; and the comparatively greater depth of the sinus results, I conceive, in 
this difference between the two sides, the smus reaching further back, or apparently 
deeper, in those which are more equilateral than where the siphonal side is so much 
the larger. Similar differences exist in J/ya truncata; what has been called 
M. Uddevallensis has one side of the shell very short, giving a material difference 
in proportional dimensions from some undoubtedly of the same species from the 
Coralline Crag, as well as from the more southern portion of the English Coast, as if 
a northern locality had induced an alteration in that character. 
I have given figures of the specimen from the Red Crag, called P. gentitis (fig. 1, 
d, e), which Mr, Sowerby considered as a distinct species. With the exception of its 
being a little more elongated, its characters are so similar to those of fig. 1, a, that I 
believe it to be only an aberrant form of the one abundant in the Coralline Crag. 
The apparent greater depth in the sinus of the mantle-mark, in this specimen, I would 
attribute entirely to its elongated and constricted form: a considerable difference in 
the siphonal scar may be observed in a large series depending in a great degree upon 
the outward form of the shell. 
Figs. 3, 4, 5, of the ‘ Mineral Conchology,’ t. 602, are probably fragments of this ; 
but figs. 1 and 2, of the same Plate, described under the name of P. Fawas, belong, I 
believe, to another species. 
Saxicava.*  Fleureau de Bellevue, 1802. 
Myritus (sp.) Linn. Mont. &c. Hratetta. Dand. 1800. Gray, 1851. 
SoLEN (sp.) Linn. Mont. &e. Ciotuo? Fauj. St. Fond. 1807. 
Mya (sp.) Linn. Fabr. &e. Byssomra. Cuv. 1817. 
Donax (sp.) Poli. Diponta. Schum. 1817. 
ANATINA (sp.) Turton. Puo.nosia. Leach, 1819. 
Carpita (sp.) Bruguiére. Biarnourus. Id. 1819. 
Inus (sp.) Oken. RuomBus. Blainv. 1818. 
CHAMPHOLAS (sp.) Lister. Aaina. Turt. 1822. 
ARCINELLA (sp.) Phil. 1844. Not Schum. Ruomsorves. Blainv. 1825. 
* Etym. Savum, a rock; and cavo, to make hollow. 
