SAXICAVA. 271 



protrudes, and the bulb-like part generally forms the inner 

 lining of a cavity, but is sometimes free. In the latter case 

 the Gastrochcena is found among madrepores and broken 

 shells, and the tubes are partially composed of the same, 

 cemented together. " The inside,^^ says Mr. Jukes in the 

 'Magazine of Natural History,^ "is smooth, and consists of 

 thin layers of the calcareous secretion applied by the animal 

 in the formation of its chamber, which somewhat resembles 

 a powder-flask; the lengthened neck through which the 

 animal passes the double tube, is formed of concentric lay- 

 ers of the same substance, preserving to a certain depth the 

 same figure as at the summit." The only British species is 

 G. Modiolina. 



Torbay on the English coast, and Bii'terbuy Bay in Con- 

 nemara, are stated to be among the most populous of its 

 habitats. "Prom the former," say the authors of 'British 

 Mollusca,' " we have taken masses of limestone well honey- 

 combed by its excavations." It is also found at Exmouth 

 and Weymouth. The other genera associated with the 

 above are — 



II. Saxicava, the animal of w^hich greatly resembles 

 that of Gastrochcena, but which never forms any supple- 

 mentary tube ; and does not always excavate, being some- 



