284 



MOLLUSCA FROM THE CRAG. 



a considerable variation in that character may be observed. Great reliance has been 

 placed npon 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. Norver/ica, 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 sinus 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 Mi/a 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. Faujas, belong, I 

 believe, to another species. 



Saxicava.* Fleureau de Bellevue, 1802. 



Mytilus (sp.) Linn. Mont. &c. 

 Soeen (sp.) Linn. Mont. &c. 

 Mya (sp.) Linn. Fair. &c. 



DoNAX (sp.) Poli. 



Anatina (sp.) Turton. 



Caudita (sp.) Bruguibre. 



IRTJS (sp.) Oken. 



Cham i;i'iiolas (sp.) Lister. 



Arcinella (sp.) Phil. 1844. Not Sehiun. 



Hiatella. Band. 1800. Gray, 1851. 

 Clotho? Favj. St. Fond. 1807. 

 Byssomia. Cuv. 1817- 

 Didonta. Schutn. 1817. 

 Pholeobia. Leach, 1819. 

 Biapholius. Ld. 1819. 

 Rhombus. Blainv. 1818. 

 Agina. Turt. 1822. 

 Rhombojdes. Blainv. 1825. 



* Etym. Saxum, a rock; and cavo, to make hollow. 



