HALF AN HOUR WITH SHELL-FISH. 181 



follow a geometrical progression. The spiral formed 

 is the logarithmic, of which it is a property that it 

 is everywhere the same geometrical curvature, and 

 is the only curve, except the circle, which possesses 

 this property. Following this law, the animal winds 

 its dwelling in a uniform direction through the space 

 round its axis. We cannot forbear quoting Mr. 

 Moseley's own remarks on this subject, which will 

 be found in a paper contributed to the " Philosophical 

 Transactions " as far back as 1838 : " There is 

 traced in this shell, the application of properties of 

 a geometrical curve to a mechanical purpose, by 

 Him who metes the dimensions of space, and stretches 

 out the forms of matter according to the rules of 

 a perfect geometry ! " There is another circum- 

 stance in connection with univalve shells we would 

 mention. It is evident that in aquatic mollusca the 

 shell must not only be a habitation for the animal, 

 but a float. This it becomes through the portion 

 of the narrower extremity of the shell being left 

 unoccupied. But in order to preserve its buoyancy, 

 and enable the animal to ascend and descend the 

 water at will, it is necessary that the increase in the 

 capacity of its float should bear a constant ratio to 

 the corresponding increase of its body. This ratio 

 always assigns a greater amount to the former than 

 to the latter. It is in accordance with the geo- 

 metrical character of the form assumed that the 

 capacity of the shell and the dimensions of the 

 animal do increase in a constant ratio, causing the 



