336 Natural History of Molluscous Aiiimah : — 



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 Pholades 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 Tubicolse, 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 Teredo, 

 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 theu* 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 (^g, 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 Teredo 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. b of Vol. 11. 



