76 UNIVALVES. 



convolutions, but at the top is depressed, and 

 has a small umbilicus ; it is usually about two 

 inches long, and one and a quarter broad at the 

 base. 



This is one of the Bullae which possess a giz- 

 zard.* In this animal it consists of two trian- 

 gular, thin, testaceous plates nearly an inch in 

 diameter, and another of an elongated semi- 

 cylindrical form. These plates are connected 

 together by a tough yellowish ligament ; and 

 form a most powerful digestive organ ; when the 

 animal has been dissected, there have been found 

 in or near this gizzard, numerous specimens of 

 the smaller testacea, with their shells reduced 

 completely to a powder. 



This is the largest species of the British 

 Bullae. 



BULL A Fontinalis^ 



STREAM DIPPER. 



Specific Character. Shell sinistral, having four 

 or five reversed volutions, glossy, pellucid,]: and 

 of a light horn colour ; the body whorl is large ; 

 the others are very small, and not much pro- 

 duced ; aperture oval oblong, three fourths of 

 the length of the shell. Old specimens are 



* Plate II. fig. 7. + Plate II. fig. 9. 



J Pellucid, so clear that the light is seen through it. From 

 Latin, per through, and lux light. 



