268 • POPULAR BRITISH CONCHOLOGY. 



united by a true ligament^ and most of tliem gape widely in 

 front. Some of the genera of this family,, as now consti- 

 tuted by Professor Porbes, enclose their bodies and shells 

 in a supplementary, tubular, shelly cavity, and compose the 

 Lamarckian genus Tuhicolcs ; and it certainly seems to me 

 more reasonable, with Lamarck, to separate the genera 

 Saxicava, Petricola, and Veuerujns, from the tube-forming 

 GastrocJmna, FistMlana, AspergiUicm, and Clavagellaj than 

 to unite them as the English professor has done. 



G-ASTROCHJENA. 



This is interesting as the only British representative of 

 the Lamarckian TuhicolcB, and as presenting an instance of 

 the desire exhibited by so many species of boring molluscs 

 to protect themselves by lining their lioles. We have al- 

 ready seen this in the case of the Teredo and some of the 

 Tliolades ; and now we come to the tuhe-maJcers, we find an 

 extension of the same principle. In the well-known Asj^er- 

 gillum, in which the tube forms the principal object, we 

 have an instance of the valves of the shell taking a rudi- 

 mentary character and forming a part of the tube. This 

 tube is formed by the edges of the valves expanding and 

 spreading laterally and perpendicularly until they become a 



