THE PEARL OYSTER. 951 



impressions are oval and irregular ; pallial impression 

 narrow and small. 



The mollusc is described as closely resembling unio. 

 The lobes of the mantle are disunited and without siphons. 

 The mouth is rather large, with a pair of palps, as in unio ; 

 and what is singular in an animal which lives adherent to 

 foreign substances, it is provided with a very large foot, 

 which may be compared, in regard of its form and position, 

 with that of unio. May not this large foot have reference 

 to the acquisition of food ? 



THE SEMILUNATE ETHERIA. 



(Etheria semilunata]. The species of the genus are at 

 present in some degree of uncertainty. Lamarck records 

 four : these M. Ferussac and M. Deshayes reduce to two ; 

 and M. Rang considers E. Tubifera of Sowerby and E. 

 Cailliaudii of Ferussac as identical ; and E. Cortoni of 

 Michelm to be the same with E. Peumbea of Ferussac. 



The variation in the form of the shell from age and 

 other circumstances is so great, that were not this fact 

 kept in view, each individual might be regarded as a dis- 

 tinct species. Even in the same species there are indi- 

 viduals armed with spines and others devoid of those 

 appendages. 



