336 Natural History of Molluscous Animals : — 
bivalve shell is so peculiar that you can seldom be at a loss 
where to place it, even at first sight. Thus, you will certainly 
not place the Phdlades amongst multivalves, as the mere 
conchologist has done, because it has a few additional pieces 
placed over and above the hinge; for the habit of these shells 
is that of bivalves, and the little accessory pieces have no cha- 
racter of proper valves. The only shells which can perplex 
you are those which Lamarck has placed in a family denomi- 
nated ‘Tubicolz, from the circumstance of the animals forming 
a calcareous tube for their protection, and which tube, until 
the French naturalist explained its true nature, had been con- 
sidered as the shell itself. To this family belong the Terédo, 
of which we have had occasion to say so much, and the Asper- 
gillum or water-pot shell, perhaps the most singular of its 
class. ‘These are truly bivalves, but the proper valves are 
small, and their existence was not recognised until lately, when 
naturalists, not satisfied with observing and admiring external 
characters, began to examine with attention internal structure. 
In Aspergillum (fg. 87.), the part generally preserved in col- 
lections is the tube, to the inside of which, near its lower 
extremity, the valves are closely soldered: but in Terédo the 
true shell is placed without the tube at the posterior extremity. 
The valves are small, and somewhat anomalous in form, while 
the tube is long, flexuose, and worm-like, and lines the bore 
which the creature has made in the wood. * 
It is unnecessary in this place to describe the various forms 
which bivalves assume, and on which their distribution into 
families and genera is founded. That will be done in a future 
letter. I wish merely further to observe, that, in relation to 
their structure, they may be divided into the compact and 
foliated. ‘The former are heavy, hard, uniform throughout, 
and have a clean fracture ; while the latter are light, divisible 
into layers, and break into irregular splints. The oysters and 
the genera allied to them exhibit this latter structure in the 
clearest manner, the pectens and muscles, both fresh-water 
and marine, less so; and, if you will compare any of these 
with a species of Mactra or Venus Lin., you will at once have 
* See the figure at p. 23. fig. 7. 6 of Vol. II. 
