MARSHALL: ADOmONS TO "BRITISH CONCHOLOGY." 207 



P. Candida var. subovata Jeff. — Paignton, S. Devon. 



var. cylindracea INIarsb. n. var. — Smaller, thin and fragile, and 

 much broader in proportion to length. L. o"5oin., b. i"5o. From 

 soft peat on Torre Abbey sands, Torquay, with a corresponding 

 variety of P. dnctyliis. 



Xylophaga praestans Smith. — Vide Journ. Make, 1903, vol. 

 v., p. 328. 



X. dorsalis Turt. — The Alder ney record is an error. I have seen 

 the shells, and they prove to be Teredo megoiara. In the same paper^ 

 the record of Teredo navalis is also an error for T. pedicellata. 



Teredinida^ Flem. — It is necessary to study the pallets of the 

 Teredines, owing to the confused manner in which several forms will 

 sometimes mix themselves up with the valves in the same piece of 

 wood. The pallet of Sowerby's figure of T. norvegica, for instance, 

 is incorrect ; it should be as Jeffreys.' The same faults attach to the 

 pallets of T navalis and T. megotara. The pallets of the latter 

 species are most variable, Jeffreys' and Sowerby's figures representing 

 the extremes, while those of its variety excisa are polymorphous. 



Teredo Sellius. — Countless remedies have been tried for over- 

 coming the destructive powers of the Teredo, or "ship worm," but it 

 still remains the worst enemy with which marine engineers have to 

 contend. Its powers in that direction may be estimated from the fact 

 that the most substantial timber erections are sometimes riddled within 

 -four to six months. Ships may be made immune by metallic sheath- 

 ing, but not timber erections. The latest method to be adopted for 

 overcoming this destruction and loss to wharves, harbours, and sub- 

 marine works generally, has been successfully carried out by American 

 contractors, who can now electrocute them by millions, and although 

 the process is not altogether permanent in its effects, yet by occasional 

 applications it is proving sufificient to overcome the difficulties experi- 

 enced in many extensive operations, and to supersede the use of 

 divers and other highly-skilled operatives. The method of electrocu- 

 tion is carried into effect by the use of a floating electric-power plant, 

 capable of generating heavy currents of electricity at a comparatively 

 low intensity. A network of wires is first lowered into the sea facing 

 the wharf or harbour to be attacked, and these are coupled with one 

 of the poles of the dynamo on the vessel ; similar wires are then sus- 

 pended beneath the ship in electrical contact with the other pole. 

 Directly the current is switched on, electrolytic action occurs in the 

 sea water between the two metal nets, and chlorine gas is thereby 

 liberated. This deadly gas envelopes the Tei-edines in their borings, 

 and speedily causes death. 



I Marquand ; Marine Shells of Guernsey, etc., Trans. Guern. Soc. Nat. Sci., 1903, Suppl. 



