90 GASTROCH^NID/E. 



in the same way that the Teredo makes the outer points 

 of its testaceous sheath. In Clavagella and Aspergillum 

 the valves are united with the case, being apparently 

 soldered to it. Mr. Tryon has lately published a careful 

 monograph on the Order ^ Pholadacea, comprising the 

 present family, Pholadidce, and Teredinidce. 



Genus GASTROCH.^'NA^ Spengler. PL III. f. 5. 



See the description of the family for that of the sohtary 

 genus which inhabits the British seas. 



Placed by Lamarck in his family Pholadaires. It 

 was included in the genus Uperotus of Guettard, Chcena 

 of Retz, Fistidana of Bruguiere, and Trapezium of von 

 MUhlfeldt. Swainson erroneously spelt the name Gas- 

 trochina. Morch, H. and A. Adams, and Tryon con- 

 sider the G. mumia of Spengler (a tropical shell) the 

 type of the present genus, and refer the European 

 species to Fleurian de Bellevue^s genus Rocellaria. The 

 only species which we possess excavates and encases 

 itself in limestone, new-red sandstone, and old shells; 

 sometimes the case is found free, and incrusted with 

 fragments of shells and grains of sand. There can be 

 no question as to its perforating powers. The case is 

 occasionally lodged in the valve of a large Pecten 7naxi- 

 mus or Lutraria eUiptica, half of it on one side of the 

 valve and half on the other. If an acid or chemical 

 menstruum were necessary for this operation, it would 

 either dissolve the calcareous matter of the envelope, or 

 not act on the uncalcareous sandstone. The shell is 

 invested by a delicate epidermis, which is more perfect 

 on the posterior or outer end, but is sometimes worn 



* Gaping in the ventral part. 



