PYRGOMA. 



TESTA compresso-subconica, vel conica, incli- 

 visa; apice pervio: basi valva testacea, modo 

 caliciformi, modo tiibulari, madreporis infixa, 

 clausa. Operculum bipartitum, valvis qjuatuor 

 compositiim. 



This Genus appears to have been instituted by Savigny, 

 and has been adopted by Leach and Lamarck: we find, 

 however, upon examining the collection of Cirripedes, in 

 the British Museum, as it now remains arranged by Leach 

 himself^ that since the publication of the Supplement to 

 the Encyclopaedia Britannica," where the characters of 

 the Genus first appear in print, he has divided it into 

 four; upon what grounds we must acknowledge ourselves 

 entirely ignorant, except it be from some differences in 

 the form of the shell, and the valves of the operculum. 

 The names which he has given to these four genera are 

 Pyrgoma^ Megatrema^ Savignium, and Aditia, We do not 

 consider them sufficiently distinct to constitute several 

 genera, nor do we think that it is for the interest of 

 science that minute differences should be exalted to the 

 rank of generic characters, wherefore we still include all 

 above enumerated under the denomination of Pyrgomdy 

 which may easily be known from all other sessile Cirripedes 

 by its upper shell consisting of only one piece, and not of 

 several laterally joined together. Like Creusia it is found 

 either sitting upon or fixed in various species of the stony 

 Corals, or Madrepores, and also in some cases entirely 

 overgrown by them: the shell is generally compressed, 

 and somewhat conical, but sometimes regularly conical, 

 open at the apex, and closed at the base by a deeply cup- 



