1896.] NATURAL SCIENCES OF PHILADELPHIA. 411 



by the usual process, of the products of secretion, to be suddenly- 

 brought in contact with alkaline salts either as dust or in solution 

 in the moisture about the animal, the result will be a sudden con- 

 traction of the portion of the mantle affected, consequently the 

 mucus deposit either will not be laid down evenly on the margin of 

 the shell or its deposition may be for the moment retarded. In 

 either case an irregularity will result. The mantle, turgescent with 

 secretion, cannot indefinitely retain the secreted fluids, and, after a 

 time, even if the alkaline irritant is still active, the mucus must be 

 exuded. But if this is done by a film of tissue, more or less irregu- 

 larly contracted, the deposition will be correspondingly irregular in 

 its location. As the epidermis is first laid down, and the more cal- 

 careous matter subsequently upon its elastic surface, it follows that 

 an irregular surface of the epidermis will be reinforced by shelly 

 matter and, as it were, petrified in its irregularity, which will be 

 exhibited permanently in the external surface of the shell. If a 

 minute process of the mantle edge would normally produce a spiral 

 thread on the surface of the shell, and its regular deposition is inter- 

 rupted by the alkalinity of dust, air or moisture about it, the tissue 

 will be obliged to contract after a short period of expansion, and the 

 spiral thread will consequently appear broken up into a series of 

 granules. The more violent the induced contraction the greater will 

 be the amount of undeposited mucus contained in the respective 

 glandular cells, and which must be got rid of at the next period of 

 expansion, and, consequently, the coarser will be the granules formed 

 by its exudation at the next opportunity. The coil of the shell is 

 determined partly by that portion already existing, against which 

 the new deposit must be laid down, and partly by the form and mass 

 of the body of the animal within the shell. The direction of the 

 coil is a resultant of the reactions between these two factors, guided 

 to a limited extent by gravity which pulls the shell, pendant from 

 the extruded animal to one side or the other, while the animal is 

 active. Yet as the deposition of shelly matter takes place chiefly, 

 if not entirely, when the animal is contracted and at rest, mostly 

 within the shell, it cannot be expected that the action of gravity 

 should have much influence on the form of the shell. But, if the 

 growth of the soft pai-ts be accelerated so that they increase in length 

 of coil disproportionately to the growth of the shell, the direction of 

 the coil is correspondingly less dependent on the form of the existing 

 whorls and more dependent on the posture assumed by the extruded 



