MOLLUSC A OR UNIVALVES. 155 



on which they live, . and preserves the same 

 form to adult age. The P. Ungarica of the 

 British Seas has the inside bright, and often of 

 a beautiful rose colour. 



Lamarck's system includes the genus An- 

 cylus amongst the Patelliform shells, but it is 

 now placed with the Lymnaidce. 



Family 6. — Bullceidce* 



The shells forming this family are very sim- 

 ple in form, but differ from the preceding in 

 some respects ; they are all slightly rolled up, 

 without being spiraL There are three genera 

 forming this family, which are described as 

 follows. 



28. Acer A. — Shell none. Animal, oblong 

 and flat ; head, not distinct ; foot very large. 

 « — 1 species, found in the Mediterranean. 



29. Bullae a. — Shell less rolled up than the 

 subsequent genus, very thin, and hidden in the 

 folds of the animal's mantle, without adhering 

 to the body. Animal, head not distinct; no 



* Les Bulleens, Lamarck. 



