218 THli MECHANICAL FUNCTIONS. 



from that which is formed subsequently. The suc- 

 ceeding turns made by the shell in the progress of 

 its growth, enlarging in diameter as they descend 

 from the apex, form by degrees a wider base.* 

 During the growth of the animal, as the body ex- 

 tends towards the mouth of the shell, its posterior 

 end often quits the first turn of the spire, and 

 occupies a situation different from that which it 

 had originally. In these cases the cavity at the 

 apex of the spire is filled up with solid calcareous 

 matter of a hardness not inferior to that of marble. 

 Such is the general form of turbinated shells. 

 It sometimes happens, however, as in the Conns, 

 that the upper surface of the spiral scarcely des- 

 cends below the level of the original portion of the 

 shell, which, in the former disposition of its parts, 

 would have been the apex : while the lower por- 

 tions of the spiral turns shoot downwards, so as to 

 form a pointed process; thus the whole is still a 

 cone, but reversed from the former, the parts last 

 formed being the outer surface of the cone, and tlie 

 circumference of the apparent base, or flat surface ; 

 while the central portion of this base is the part 

 which was first formed. 



* Mr. Moseley has shown that the increase in the size of the 

 whorls, and also of the distance between adjacent whorls, takes 

 place according to a geometrical progression ; and that, as conse- 

 quences of the observance of this law, the form of the curves gene- 

 rated is in all cases that of a logarithmic spiral, and the angle at 

 which the curve intersects the radius vector is constant. He has 

 verified the accuracy of this law by actual admeasurement in a great 

 number of cases, both of turbinated and of discoid shells, and has 

 deduced from it sonne interesting conclusions relative to the increase 

 in the powers of vitality as the animal advances in growth. See 

 Philosophical Transactions for 1838, p. 351. 



