* _Tellìna_, wedge-shell, ex. _T. Feroensis_.
* _Mactra_, kneading-trough, ex. _M. stultòrum_.
* _Donax_, wedge-shell, ex. _D. trunculus_.
_Spondylus_, thorny-oyster, ex. _S. gæderopus_.
* _Anomia_. antique lamp, ex. _A. ephippium_.
* _Pinna_, wing-shell, ex. _P. pectinàta_.
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_.
* _Buccinum_, whelk, ex. _B. reticulatum_.
* _Strombus_, screw, ex. _S. pes-pelicàni_.
* _Trochus_, top-shell, ex. _T. bifaciàtus_.
* _Haliòtis_, ear-shell, ex. _H. tuberculàta_.
2nd Section, without a regular spire: 5 genera.
* _Dentàlium_, tooth-shell, ex. _D. elephantìnum_.
* _Sérpula_, worm-shell, ex. _S. triquétra_.
Total number of genera in the arrangement of Linnæus, thirty-six.
The genera marked with an asterisk, contain species found in Britain.
"Multivalves may be divided into two kinds, the _pedunculated_, or those fixed to other bodies, as rocks, stones, planks, &c.; or _free_, as the _Chiton_ and _Pholas_.
"Our first genus is _Chiton_. The shell is easily known. The fixed character is, many valves placed over each other along the back. I have already noticed the membrane which connects the valves, which is also a permanent character. It is elastic: the sides are either scaly, as in _C. squamòsus_, (Plate 1,) and hairy, or spinous. The species are determined by the margins. Some of the Chiton genus are common upon our own coasts; they are frequently found among seaweed and stones, rolled up like a ball. _C. fasciculàris_ and _C. lævis_ are British; there are some other British species. The animal adheres to rocks, like the _Patella_, or limpet. The number of species forty.
"Second genus, _Lepas_. Shell multivalve, fixed at the base; valves erect, or _upright_.
"Observe how much the situation of the valves differs in _Chiton_ and _Lepas_. It is scarcely possible to mistake the one for the other. The feathery tentacula, or feelers, of _Lepas anatifera_ are worthy notice, and in a state of motion must be yet more beautiful. (Plate 2.) The common acorn-shell, _L. balanus_, is to be seen very frequently upon the shells of muscles, oysters, periwincles, whelks, in large groups. I see that you are examining the different appearance of the pedunculated and the sessile _Lepades_. Linnæus made two divisions; later writers have separated them into several distinct genera, which will be noticed when we attend to Lamarck's system. The species are forty-five, of which several are found on the British shores, as _L. tulipa_, _L. diadema_, _L. tintinnabulum_, _L. balanus_, _L. anatifera_, &c. The Indian, American, and Atlantic oceans afford numerous species.