310 SUPPLEMENT 
but the manner in which it is rolled, from front to rear in one 
and the same vertical plane, so that it may be cut into two 
completely similar parts, is a character without any parallel 
in the living state, except in the spirula. Itis the same with 
its concameration, or the division of the posterior part of its 
cavity into a great number of lodges, formed by simple or 
smooth partitions, convex behind, concave in front, and at 
the middle of each is an orifice, which is prolonged into the 
internal of the following partition. These partitions, and the 
chambers which they form, are evidently produced by the ad- 
vance of the animal, determined by its growth, during the 
whole active duration of its life: something of the same sort 
takes place in ‘certain spiral shells. The siphon is evidently 
the point of attachment of the terminal muscle of the body, 
analogous to the columellary muscle of the same spiral shells, 
and which necessarily follows the progress of the entire ani- 
mal. The animal itself is only enclosed in the first lodge, 
whose size and even form have no resemblance with those of 
the other abandoned lodges. From all this it may be con- 
ceived that the number of the chambers and their proportional 
extent vary in each individual. In regarding the composition 
of this shell we find it to be of two parts, one exterior, of but 
little thickness, of the usual nature, coloured by irregular 
bands, transverse, reddish or ochreous on a white ground, 
which becomes more and more predominant as we approach 
the aperture, and of an internal stratum, thicker, and nacreous. 
This stratum spreads even over the back, from the return of 
the spire to the interior, and more or less externally, so as to 
close more or less completely the two lateral umbilici situated 
at the two extremities of the transverse pillar. It is upon 
this nacreous part that may be easily seen, both to right and 
left, and pretty deeply, two broad muscular impressions, 
rounded, and which tend to unite together. 
We have said above that this shell has evidently many re- 
