"The animals of the _Mollusca_ are soft, without joints, generally possessing a head, eyes, and tentacula, or feelers. They have also a fleshy membrane, called a foot, which they use for climbing. The orders, excepting the first, are named from the position of this foot. They are five in number. The first order contains very few genera. One genus, named _Cleodora_, contains a species brought from Africa. The shell is curious, transparent, and shaped like the head of a halberd. "The animals of the second order, _Gasteropoda_, have a muscular foot, or disk, on which they rest. The families are seven. The first, _Tritonia_, I shall pass over. "_Phyllidiana_ includes the genera-- _Phyllidia._ _Chitonella._ _Chiton_ Coat of mail. _Patella_ _Patella_, or limpet. [Sidenote: CHITON. PATELLA.] "The _Chiton_ moves like the _Patella_, upon a disk, or foot. "The body of _Patella_ is entirely covered by the shell. You may have many opportunities of examining the British species. _P. pellucida_ is very transparent, with blue lines. * * * * * "The family _Semi-phyllidiana_ contains _Pleurobranchus._ _Umbella_ _Patella_. "The _Umbella_ shell is flat and white, and is sometimes four inches in diameter. It is common in the Isle of France: there is another from the Mediterranean. * * * * * "_Calyptracea_ is a larger family: it has many genera taken from _Patella_: _Parmorphorus_, Thracian-shield. _Emarginula_, } _Fissurella_, } _Pileopsis_, } _Patella_. _Calyptræa_, } _Crepidula_, } _Ancylus_, } [Sidenote: FISSURELLA. PILEOPSIS. BULLA.] _Parmophorus_ is found in the seas of New Holland and New Zealand. The margin of the next genus is distinguished by a slit: the shell is conic. (Plate 3.) _Fissurella_ has the top of the shell perforated; it is called the _key-hole_ limpet, from the shape of the aperture. _Pileopsis_ is obliquely conic. It was with this division that the curious _Hipponyx_ ranked. "_Calyptræa_ is very thin and brittle, with an internal lip. (See Frontispiece.) "_Crepidula_ has the shell partly covered, or arched: it looks like a little slipper. "_Ancylus spina-rosa_ is a pretty species from the south of France: the genus is fluviatic. _A. lacustris_ and _fluviatilis_ are both natives of our fresh-waters. * * * * * "The next family, _Bullæana_, has _Acera_ _Bulla_. _Bullæa_ _Bulla_. _Bulla_ The same.