324, MAZATLAN UNIVALVES 
and smooth, with the mouth at right angles. The successive 
portions are not formed by uniform progression; but often a 
very sudden turn is seen in the arcuation, or rapid thickening 
of the body. The most singular state is when the thick annu- 
lar part is just beginning. Then at the end of a smooth slender 
tube we have a bowl with three or four rings outside suddenly 
rising at a marked angle, not unlike a short tobacco pipe. The 
rings are often nearly evanescent, when the shel] can hardly 
be distinguished from the next species. It is found of very vari- 
able lengths in its growing state; perhaps from the decadent 
portion not always falling off immediately after the new part 
is formed.* The greater or less prominence of the plug appears 
due to the same cause. The sharply triangular outline of its 
profile is characteristic of this and the next species. The shell 
in its adolescent state would appear to stand at a considerable 
angle to the plane of the animal’s foot: but when adult, it is well 
fitted to make its way along the narrow worm-eaten galleries 
in which it is generally found. The mouth is then so slanting 
that the two truncations of the cylinder form an angle of about 
115° to each other. This is contracted at each extremity ; the 
contracted part not being waved. The mouth, after being 
very much narrowed, makes a slight sharp expansion all round, 
like the neck of ajar. The texture of the shell is sometimes 
like alabaster, sometimes slightly waxen, perhaps from the 
epidermis. This appears to be the common species at Mazat- 
lan, as C. firmatum is at Panama, about 320 specimens alto- 
gether having been found: of these 37 fortunately possessed 
the operculum. This tiny body, which in an adult shell is only 
‘013 across, nearly filling the constricted aperture, is very con- 
eave externally, with a central nucleus and about 15 whirls, 
which are for the most part strongly marked, with an entire 
margin. The youngest among the specimens which were 
found perfect measures long. (042, lat. ‘01. 
A sp. (form monstrosum) 5 Ob) & BOL=—02: 
An adult pe Ae MO FA CBes 1 2ODTE 
* The very complex nature of the plug in all the Ceca, apparently formed by 
suecessive layers like ordinary shell, and not of uniform material like the septa 
in Turritella, Vermetus, &e., taken in conjunction with the variable length of 
specimens apparently of the same age, leads to the conjecture that the same plug 
may be carried onwards during the life of the animal ; that after making a fresh 
length, it loosens the plug, (adding to its margin,) and fixes it in its new locality. 
Perhaps at the same time it disengages the old portion; as the edges are always 
regular, differing from Melaniz, Truneatelle, &c. which rot off of themselves, 
and the shell is never found with the disused portion adhering. A second Clark, 
keeping a Vivarium of Ceca in an old Spondylus, might reveal charming anec- 
dotes of the habits of these creatures! 
