the Pholadidie into Natural Groups. 381 



9. Talona. Front gape small ; dorsal valves two, moderate, 



diverging. 



B. Dorsal muscles only covered with a horny or coriaceous epi- 



dermis. The cavity in which they live not lined with a regular 

 shelly tube inclosing the valves. Zirfseina. 



10. Zirf^ea. Dorsal plate not reflexed over the umbo. 



11. Navea. Dorsal plate produced and reflexed over the umbo. 



C. Dorsal muscles only covered with a coriaceous epidermis. Cavity 



in which they live lined with a regular shelly tube surround- 

 ing the valve. Teredinina. 



12. Teredo. Tube subcylindrical, hard, shelly ; pallettes simple, 



oblong or transverse, entire. 



13. Xylotrya. Tubes subcylindrical, hard, shelly ; pallettes 



elongate, pennate. 



14. Cuphus. Tube subcylindrical, thick, shelly; apex divided 

 into two separate tubes ; pallettes . 



15. Guetera. Tube club-shaped, thicker, and closed beneath ; 

 pallettes ovate dentate. 



A. Dorsal muscle protected by one or two shelly dorsal valves. 

 Cavity in which they live not lined with a regular shelly tube, 

 Pholadina. 



The upper part of the cavity in which these animals reside is 

 sometimes furnished with a calcareous deposit, which does not 

 assume the regular form of a shell, but appears to be only an 

 irregular exudation from the surface of the tubes, and never in- 

 closes the shelly valves, and it has only been observed in one or 

 two genera, as Martesia, where it is formed of hard uniform cal- 

 careous matter (see Gray, Phil. Trans. 1833 ; Rang, Man. 345 ; 

 Sow. Thes. Conch.), and Xylotrya, where it appears to be formed 

 of agglutinated particles of sand : this latter was first shown to 

 me by Mr. Edward Charlesworth. 



a. Siphons naked at the base ; front gape of the shell more or less 

 large and open, never closed with an additional shelly plate. 



1. Pholas, Linn., part. ; Gray, 1849. 



1. P. costatus, Linn., Sow. Thes. t. 102. f. 8, 9. 



2. P. crucifera, Sow. Thes. t. 104. f. 24, 26. 



3. P. truncata, Say, Sow. Thes. f. 29, 30. 



4. P. latissima, Sow. Thes. f. 15, 16? 



2. Barnia. Barnia, Leach MSS. Barnea, Risso. 



1. B. australasise. Ph. a., Gray in Sow. Thes. t. 107. f. 73. 



