"The three principal divisions you are already acquainted with, namely, _Multivalves_, shells composed of several parts called valves; _Bivalves_, formed of two parts; and _Univalves_, composed of one part or piece only. "These divisions contain several _genera_, and the genera usually include many individual shells, but occasionally only one. "The characters of every genus are permanent, and are therefore to be observed in every one of the species contained in the genus. "Species are determined by shape, colours, or appearances on the surface of the shell: there are sometimes many varieties of the same species. [Sidenote: LINNÆAN GENERA.] "The Linnæan genera are thirty-six. I have copied a list of them for you. In that tray are the specimens mentioned in your list: ARRANGEMENT OF LINNÆUS. First Division--_Multivalves_: 3 genera. * _Chiton_, coat of mail, example, _C. squamosus_. * _Lepas_, acorn-shell or barnacle, ex. _L. anatifera_ and _tintinnabulum_. * _Pholas_, stone-piercer, ex. _P. candida_. Second Division.--_Bivalves_: 14 genera. * _Mya_, trough-shell, ex. _M. truncàta_. * _Solen_, razor-sheath, ex. _S. siliqua_. * _Tellìna_, wedge-shell, ex. _T. Feroensis_. * _Cardium_, cockle, ex. _C. cardissa_. * _Mactra_, kneading-trough, ex. _M. stultòrum_. * _Donax_, wedge-shell, ex. _D. trunculus_. * _Venus_, Venus, ex. _V. Paphia_. _Spondylus_, thorny-oyster, ex. _S. gæderopus_. _Chama_, clamp-shell, ex. _C. gigas_. * _Arca_, ark-shell, ex. _A. Noæ_. * _Ostræa_, oyster, ex. _O. isognomon_. * _Anomia_. antique lamp, ex. _A. ephippium_. * _Mytilus_, muscle, ex. _M. edulis_. * _Pinna_, wing-shell, ex. _P. pectinàta_. [Sidenote: LINNÆAN GENERA.] Third Division: 2 sections.--1st. _Univalves_ with a regular spire: 14 genera. _Argonauta_, paper-sailor, ex. _A. argo_. _Nautilus_, sailor, ex. _N. pompilius_. Conus, cone, ex. _C. Hebræus_. * _Cypræa_, cowry, ex. _C. monèta_. * _Bulla_, dipper, ex. _B. naucum_. * _Voluta_, wreath, ex. _V. utriculus_. * _Buccinum_, whelk, ex. _B. reticulatum_. * _Strombus_, screw, ex. _S. pes-pelicàni_. * _Murex_, rock-shell, ex. _M. ramòsus_. * _Trochus_, top-shell, ex. _T. bifaciàtus_. * _Turbo_, wreath, ex. _T. muricàtus_. * _Helix_, snail, ex. _H. nemoràlis_. * _Nerìta_, nerite, ex. _N. striàta_. * _Haliòtis_, ear-shell, ex. _H. tuberculàta_. * * * * * 2nd Section, without a regular spire: 5 genera. * _Patèlla_, limpet, ex. _P. vulgàta_. * _Dentàlium_, tooth-shell, ex. _D. elephantìnum_. * _Sérpula_, worm-shell, ex. _S. triquétra_. _Terèdo_, ship-worm, ex. _T. navàlis_. * _Sabella_, Sabella, ex. _S. Belgica_. Total number of genera in the arrangement of Linnæus, thirty-six. The genera marked with an asterisk, contain species found in Britain. [Sidenote: CHITON. LEPAS.]