258 GROWTH OF THE SHELL chap. 



return to Marseilles, an Avicula 78 mm. and an Ostrea 95 mm. 

 long (both being species peculiar to W. Africa) were taken from 

 its keel. These specimens had therefore attained this growth 

 in at most 154 days, for at the period of their first attachment 

 they are known to be exceedingly minute. P. Fischer relates ^ 

 that in 1862 a buoy, newly cleaned and painted, \vas placed in 

 the basin at Arcachon. In less than a year after, it was found 

 to be covered with thousands of very large Mytilus edulis^ 100 

 mm. X 48 mm., the ordinary size on the adjoining banks being 

 only about 50 to 60 x 30 mm. 



Some observations have already been recorded (p. 40) on 

 the growth of Helix aspersa. In the summer of 1858, which 

 was very dry, especially in the south of France, the young 

 Helices born that year were still very small in August. About 

 the end of that month abundant rain came on, and in four or five 

 days young H. variabilis, H. pisana, and H. aspersa^ Q2iimg with- 

 out cessation, as if to make up for lost time, grew more than a 

 centimetre of shell. The lip of a young H. arhustorum has been 

 observed to have grown, at the end of the first week in the season's 

 growth, 3 mm., at the end of the second week, 6-25 mm., the 

 third, 11-5 mm., and the fourth 12*5 mm., with a finished lip.^ 



Careful observation has shown that in the growth of the shell 

 of Helix aspersa the periostracum is first produced ; it is covered 

 with hyaline globules, 10-12 mm. in diameter, which persist 

 even in the oldest shells. Calcareous matter is deposited on the 

 internal face of the new periostracum, at some distance from the 

 margin. It is secreted by a white zone or band of cells bound- 

 ing the entire breadth of the mantle as applied to the peristome. 

 Immediately behind the white zone are a series of pigment cells 

 which not only give the shell its colour but complete the calcifi- 

 cation of the shelly matter laid down by the white zone. When 

 the animal has attained its full growth and the lip is finished 

 off, the white band and the periostracum cells completely dis- 

 appear, and only such cells persist as contribute to the internal 

 thickening of the shell. Shell growth, in this species, is very 

 rapid. If a portion of the pulmonary sac is laid bare, by remov- 

 ing a fragment of shell, at the end of 1^ or 2 hours there may 

 be detected a delicate organic membrane covering the hole, and 

 strewn with crystals of carbonate of lime. This thickens Avith 

 1 Journ. de Conchyl. xii. p. 3. 2 -p. Scott, Journ. of Conch,, 1887, p. 230. 



