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. 6 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 (/evis, Penn.), or at least without any distinct 
longitudinal ribs or striee, 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. 8 may be called lenticular, with numerous fine rays covering the entire surface, 
sometimes single, sometimes in pairs; var. y has four or five raised ribs, as before 
described ; var. 6 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 striz, 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 evolefus, 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 1} 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. 
