L70 



younger the shell the wider is the anterior gape. When the bore is com- 

 plete and the Pholas, safely lodged in the wood or stone, arrives at ma- 

 turity, the boring functions of the foot are dispensed with, and it termi- 

 nates its building operations by closing in nearly the whole of the gape 

 with shell matter. The Pholas papyracea (Pholadidea, Forbes and Han- 

 ley) does this ; but in the foreign P. striata and its allies the peculiarity is 

 much more conspicuous. 



We have now to notice the modifications of the hinge plates. In the 

 large P. costata of Linnseus, regarded as the type of Pholas proper, there 

 are two plates, one anterior and one posterior ; in the British P. dactylns 

 (Bactylina, Gray) there are two oblong plates side by side with a small 

 transverse plate, and a long lanceolate plate behind them ; the British P. 

 Candida and parva represent another section [Barnea, Bisso) in which the 

 hinge is capped with only a single plate in the form of an umbonal shield ; 

 in another section, represented by the British P. crispata [Zirphcea, Leach), 

 the accessory plates are wanting, and their place is supplied by a more 

 dense secretion of the anterior marginal reflection, covered by a conspicu- 

 ous epidermic membrane ; in P. subglobosa and two others, small species 

 inhabiting sponge, the hinge plate is in a single piece, reflected over the 

 umboes ; the British P. paptyracea [Pholadidea, Turton) is the type of a 

 section in which the hinge plates are almost obsolete, minute, and of a 

 rhomboidal form, the anterior gape of the shell is walled in at maturity 

 by a thin layer of shell, and a horny cup is formed at the posterior end 

 for the protection of the siphons which, in this species, are framed with a 

 star-like disk at the extremity ; P. explanata [Talona, Gray) forms a horny 

 beak at the posterior end, and the valves have a curiously winged trans- 

 verse plate against their abruptly terminated anteriorly reflected edges; 

 another section, of which there are four species, is represented by the 

 curious P. Cumingii (Jouanneiia, Gray), with the left valve overlapping 

 the right valve, the front gape closed in at maturity, and a single dorsal 

 plate ; P. Janellii (Pharapholas, Conrad), with two dorsal plates, also 

 closed in anteriorly and curiously divided obliquely by a groove on each 

 side, represents another small section; P. striata (Martesia, Leach), also 

 divided by a groove on each side, closed in anteriorly, with the addition of 

 on one side a lanceolate, on the other a peltate plate, represents a more 

 numerous section, sometimes inhabiting brackish water; and, lastly, the 

 little globose P. dorsalis (Xylophaga, Leach) is the representative of a sec- 

 tion in which the shell approaches that of Teredo divested of its siphonic 

 tube, and has two little wing-like hinge plates. 



The geographical distribution of the P ho lades is almost world-wide. 



