28 MOLLUSCA FROM THE CRAG. 



variation as is exhibited by those obtained in our own seas at the present day. In the 

 Red Crag, specimens are much more scarce, although var. 8 I have seen only from 

 this Formation. 



From the great diversity of forms displayed by this animal, it is not to be 

 wondered at, that it should have been separated into several species : with variations 

 exceeding in appearance what are generally considered as sufficient for specific dis- 

 tinction, there is no permanent character that will justify their separation. In the 

 Crag as well as in the recent state, specimens may be obtained presenting every 

 minute gradation, and these apparently different forms are now justly included under 

 one name. 



In var. a the shell is nearly smooth (lavis, Penn.), or at least without any distinct 

 longitudinal ribs or strise, or with the edges only presenting these radiations ; this 

 may be considered as one extreme of the species, while the other variety exhibits five 

 large elevated ridges or ribs ; these are generally separated, and in pairs, though they 

 are sometimes simple, the intermediate spaces are filled up with three or more rays. 

 Var. /3 may be called lenticular, with numerous fine rays covering the entire surface, 

 sometimes single, sometimes in pairs ; var. 7 has four or five raised ribs, as before 

 described ; var. S has from seven to nine rays, which are themselves faintly striated, 

 while the intermediate spaces are also filled with radiating lines ; but these forms are 

 not permanent, and specimens uniting some of the characters of each may be commonly 

 obtained. Every specimen is strongly marked with fine divaricating striae, in a curving 

 direction from the umbo to the sides. In the young state the shell is often very scabrous, 

 the rays being covered with regularly raised imbrications. The general form may be 

 described as suborbicular, though the greater diameter is from the umbo to the ventral 

 margin. The auricles may be considered its most distinguishing character ; these are 

 very unequal, the posterior one small, nearly obsolete, while that on the anterior side 

 is large, generally costated, or coarsely rayed. In the early stages of its growth, the 

 shell is always regularly convex or lenticular, but in some specimens, when at a certain 

 size, its form is altered by the enlargement of the shell on the inner edge of the margin 

 instead of outwardly, so as to give a greater space to the interior without much in- 

 creasing the diameter of the shell, and this character is exhibited in both valves. 

 Fig. 2, g, is what in my Catalogue was enumerated with doubt as a new species, under 

 the name exoletus, but which probably is only a giant monstrosity of this species, as 

 the ordinary form is well displayed on the outside, beyond which, by apparently an 

 extra effort of growth, it has induced a deposit of calcareous matter, till it has 

 reached a diameter of U inches, thus much exceeding the general size of this species, 

 which is rarely more than one inch. Fig. 2, a, is from a specimen belonging to Mr 

 Bridgman, obtained in the Mammaliferous Crag, near Norwich. 



The animal in the recent state is said to range from 12 to 60 fathoms, while 18 or 

 20 is its most favorite depth. 



