317 



PHOLADID^E. 



PHGLADIDJ2. 



319 



4. Pholai, Linn., with the sub-genera PJiolai, Linn. ; Pholidoea, 

 Leach ; Martesia, Leach ; and Xylophaga, Sow. 



5. Teredo, Linn., with the sub-genera Teredo, Linn., and Teredina. 

 (' Malacology,' 1840.) 



The geographical distribution of the Pholadef is very wide, and their 

 habit of boring bard substances, such as indurated mud or clay, wood, 

 and stone, renders them, as well as other terebrating testaceans, an 

 object of anxious interest to those who construct submarine works. 

 The Breakwater at Plymouth was soon attacked by the Pholades, 

 and in Dr. Goodall's fine collection, which has since been sold, there 

 was a specimen from the Breakwater perforated by these testaceans. 

 Wood is also attacked by this genus, and submarine piles are conse- 

 quently exposed to their ravages. To counteract their operations in 

 the latter substance, nails closely driven into the submerged part of 

 the timber, as in the piles which support the pier at Southampton, 

 ieetn to be the best safeguard hitherto applied. When unmolested, 

 the young Pholadtt excavate burrows in the substance which chance 

 has opposed to them, or to which choice for it in not improbable that 

 the young shell-fish may in tome cases have the faculty of making the 

 selection of the material in which it is to pass the whole of its life 

 has directed it. 



The accompanying cuts will convey some notion of their ravages 

 upon the substances which they penetrate : 



A block of itonc perforated by Pkolas dactylui. 



(Xylophaya] dwialis in wood. 



The mode in which the operation of boring is conducted has been 

 frequently discussed. From the observations of M. Cailliaud it would 



appear that the valves are quite equal to the task of boriug limestone. 

 [LiTHOPHAGiDjE ; CtAVAOELLA ; GASTROCH.ffiNA.J Mr. G. B. Sowerby, 

 in his description of Pholas acuminata, found by Mr. Cuming at 

 Panama in limestone at low water, notices one specimen in that gen- 

 tleman's collection as demonstrating a fact of considerable importance 

 to geologists : it is in argillaceous limestone, very much resembling 

 lias, and, in forming the cavities in which it resides, it has, by such 

 chemical process as in Mr. Sowerby's opinion frequently takes place, 

 absorbed a much greater quantity of the rock than could be retained 

 or converted ; this is again deposited at the upper part of the cavity, 

 and thus the rock is recomposed. (' Zool. Proc.,' 1834.) 



Pholas has been found at depths varying from the surface to 17 

 fathoms, and Xylophaga from the surface to 45 fathoms. 



The species are numerous, and some are very abundant on our own 

 coasts. " Of these," says Mr. G. B. Sowerby, " Pholas crispata, 

 P. dactylui, P. Candida, and P. parra, are the most common ; several 

 others are described by Turton in his ' Briflsh Bivalves,' of which we 

 are quite convinced the P. lamellata is only the young of P. papyracea : 

 we are not acquainted with his P. tubermlata. Much confusion appears 

 to prevail in regard to several very distinct species : among these we 

 believe the P. papyracea of Turton is the P. itriata of Montagu ; the 

 P. davataol Lamarck is the P. itriata of Linnseus, but not of Mont." * 

 The number of living species of Pholai noted by M. Deshayes in his 

 Tables is 15, and of these one, P. Candida, is recorded as living and 

 fossil. Woodward, in his ' Manual of the Mollusca,' gives 25 recent 

 species of Pholas and 25 fossil. Forbes and Hanley give the following 

 as British species : P. dactylus, P. parva, P. criapata, P. Candida, 

 Xylophaga donalis, Pholadidea papyracea. 



The following cuts, with those previously given, will show the variety 

 of form put on by the shells of this genus : 



Xylvphaga dorsal is. 



a, enlarged view of interior ; i, dorsal view, natural size ; c, ventral view, 

 natural tize. 



This species, which is found in cylindrical cavities eaten (!) in wood 

 (see cut in previous column), bears some resemblance to Teredo, but is 

 without the shelly tube, nor has it the posterior hiatus. 



Fholat (Pliolailidta) papyraccus. 

 a, side view ; It, dorsal view j c, cup-itmpcd membrane. 



Teredo is the name given by Linnaeus to a genus of Testaceous 

 Molluscs, highly interesting on account of the ravages which one of 

 the species commits upon submerged wood. 



Linnaeus, in the twelfth edition of the ' Systema Naturae," placed 

 the genus among his Vermet Teitacea, between Serpula and Sabella. 



Cuvier, in his last edition of the ' Regne Animal,' makes Teredo one 

 of the genera of his Enfenne's, the fifth family of his Acephalous 

 Testaceans, arranging the genus between Pholai and fistttlana. 



The following are its characters : 



Animal very much elongated, vermiform, with a very delicate mantle, 

 open in front and at its lower part for the passage of a mammiliform 

 foot; tubes separated, very short, especially that for the dejections; 

 mouth small ; labial appendages short ; anus situated at the extremity 

 of a small tube floating in the cavity of the mantle ; branchiu; riband- 

 like, united on the same line throughout their length, and a little 

 prolonged in tho siphon ; a muscular ring at the point of junction of 

 the mantle and the tubes, in which is implanted a pair of pediculated 

 corneo-calcareous appendages or palmules, playing laterally one against 

 the other. 



Shell rather thick, very short, annular, equally open before and 

 behind ; equivalve, inequilateral, angular, with triangular valves, 

 trenchant in front, and only touching each other by the two opposite 

 edges ; no hinge ; an elongated, nearly straight, subfiliform, spoon- 

 shaped process ; a single slightly-marked muscular impression. 



* Zoologist* sometimes make tbe word ' Pholas ' masculine, sometimes femi- 

 nine. Llnneeus makes it masculine, and so it should be considered. 



