"Lamarck founds his system upon the structure and form of the animals, so far as they have been ascertained, and with which the exterior, or shell, must necessarily coincide. The conchology occupies the three last _classes_, and _one order_ of another class, in the well-known work which I have before mentioned to you. [Sidenote: ANNULARIA. SEDENTARIA.] "To begin with the 3rd Order of the 9th Class:-- Class, _Annularia_. Order, _Sedentaria_, Annulated Worms. Lamarck. Linnæus. _Siliquaria_, taken from _Serpula_. _Dentalium_, ---- _Dentalium_. _Pectinaria_, ---- _Sabella_. _Sabellaria_, ---- _Sabella_. _Spirorbis_, } _Serpula_, } ---- _Serpula_. _Vermilia_, } _Galeolaria._ _Magilus._ "There has been much variation in the opinions of naturalists respecting the proper place of genus _Dentalium_. Cuvier, a very celebrated writer, agreed nearly with Lamarck; but still more recently it has been considered as nearest to a new genus, _Fissurella_, (_Patella_.) The fossil-shells are found in London clay in great numbers; in the marle at Folkstone, &c. "_Spirorbis._ All the species are minute, fixed upon sea-weeds, and other marine substances. The animal which inhabits them is of a deep red colour. "_Galeolaria_ is a New Holland genus. "_Magillus_ is found in the Isle of France; the shell is sometimes three feet in length. * * * * * [Sidenote: CIRRHIPEDA--FIRST ORDER.] "Class 10th. _Cirrhipeda_, contains two orders: the first, _sessile_, or placed upon some other body; the second, _pedunculate_, and fixed at the extremity of the pedicle to other substances. "The class takes its name from the _Cirrhi_, or feathery tentacula. The genus _Lepas_ only is contained in the _Cirrhipeda_ class. * * * * * "1st Order. Shells Sessile. Genera. _Tubicinella._ } _Coronula._ } _Balanus._ } _Acasta._ } All included in _Lepas_. _Creusia._ } _Pyrgoma._ } "The first genus contains but one species; the shell is buried up to its aperture in the skin and fat of whales. "The second, _Coronula_, is found inserting itself in the sea-turtle, &c. "_Balanus_ is known to you as the acorn-shell; a genus widely diffused; abounding on rocks, shells, and wood, in large colonies. [Sidenote: CIRRHIPEDA--SECOND ORDER.] "_Acasta_ is found upon sponge. "For examples of _Creusia_, we must examine our _madrepores_, and other corals; the shells of this genus are either affixed or buried in them. "_Pyrgoma_ likewise adheres, or penetrates into corals. "In the 'Penny Cyclopædia,' under the word _Cirrhipeda_, you will find much useful information, and some plates that will give you a good idea of this class. In the British Museum you may see many of the species, and may thus make yourselves familiar with them.