PHOLADID^E. 169 



fine pectinations ; the pairs of palpi, which are pendent from 

 each side the buccal orifice, and united around it by a mem- 

 brane, are paler than the branchiae ; they are long, triangular, 

 pointed, of fine texture, and delicately striated. All the My<R 

 exsert from the anal siphon the usual tubular hyaline valve. 



This species inhabits the low- water levels of the littoral 

 zone, in the loose sandy districts, and are enabled, by the 

 powerful foot and siphonal sheath, to bury themselves more 

 than a foot in depth. It is also taken in the coralline zone at 

 Exmouth. 



PHOLADIDJB. 



Pholas, Teredo, and Xylophaga constitute the British ge- 

 nera of this family. I have submitted the Pholades and the 

 Teredo megotara to a close examination, not only of the ex- 

 ternal organs, but I have entered into detailed observations on 

 their anatomical structure. Grave errors exist in our records 

 relative to this family, both as regards the shells and the 

 functions of the soft parts. 



It is really strange that in so celebrated and ancient a genus 

 as Pholas, so often the theme of discussion, so many doubts 

 and contradictory accounts should still prevail respecting the 

 hinge, cartilage, ligament, and adductor muscles of the animal. 

 Though there may be many errors, I think that malacologists 

 will find, in this account, some rectifications, obscure points 

 explained, a variety of new matter, and that the observations 

 on the structure of P. dactylus and Teredo megotara will assist 

 not only to illustrate this family, but, mutatis mutandis, give a 

 general view of the material points of the organization of the 

 animals of the Acephala. 



As I have, in the accounts below, entered so largely into 

 the incidents of this group, I shall at once plunge " in medias 

 res." 



The first animal has caused great discussion, whether it 

 ought to form two distinct species or only one, by considering 

 its very different aspects as identical. We think it will 



