IV.] INTRODUCTION. IxV 



unmercifully pounded alive for that purpose. Even in 

 the present day snail-broth is said to be serviceable as a 

 lenitive. In France an extract from snails, called ' He- 

 licine/ is used in similar cases. The spongy plate of 

 the common cuttle [Sepia officinalis), calcined oyster- 

 shells, and the Limacella of the large slug [Limax maxi- 

 mus), as well as "crabs^-eyes^^ (or the concretions of 

 calcareous matter found in the stomach of the eatable 

 crab), were used during the last century in the prepara- 

 tion of certain medicines. 



[Ornament.) — When the Romans were the masters of 

 these islands they ransacked not only our seas and estu- 

 aries for oysters (those from the Mediterranean being 

 very scarce and inferior in quality) but also our northern 

 rivers for pearls, which were extracted from the Unio 

 margaritifer. This shell, with its accompanying product, 

 is represented in the Frontispiece to the present volume. 

 The search for native pearls continued until a compara- 

 tively modern period, when it was superseded by the 

 successful prosecution of the true pearl-fishery in Eastern 

 seas, the valuable and lustrous produce of which threw 

 into the shade our comparatively worthless and dull 

 jewel. Other species of freshwater mussel, as well as 

 the oyster, ormer, sea- mussel, and cockle, and even the 

 periwinkle, occasionally yield pearls, but of an unservice- 

 able kind. It is evident, from an examination of the 

 shells in which such excrescences are formed, that they 

 are o\ving to an irregular and partial secretion by the 

 mantle of the nacreous and lamellar substance which 

 lines the inside of the shell. In all probability the 

 proximate cause is some extraneous body, and not dis- 

 ease as was formerly supposed. In freshwater bivalves 

 the irritating tenacity of parasitic insects and worms 

 [Limnochares Anodontce and Distoma duplicatum), and 



