PHOLADIDZ. 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 branchiz ; they are long, triangular, 
pointed, of fine texture, and delicately striated. All the Mye 
exsert from the anal siphon the usual tubular hyaline valve. 
This species inabits 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. 
PHOLADID. 
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, m 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 
