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 shell 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 a jar. 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- 

 cave 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) '067, '01 '02. 



An adult '074, '021. 



* The very complex nature of the plug; in all the Caeca, apparently formed by 

 successive layers like ordinary shell, and not of uniform material like the septa 

 in Turritella, Vermetus, &c., 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 Melaniae, Truncatellae, &c. which rot off of themselves, 

 and the shell is never found with the disused portion adhering. A second Clark, 

 keeping a Vivarium of Caeca in an old Spondylus, might reveal charming anec- 

 dotei of the habits of these creatures ! 



