Whittock, Nathaniel: The Art Of Drawing And Colouring From Nature, Flowers, Fruit, And Shells: To Which Is Added, Correct Directions For Preparing The Most Brilliant Colours For Painting On Velvet, With The Mode Of Using Them, Also The New Method Of Oriental Tinting ; With Plain And Coloured Drawings (London, 1829)

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It is not the province of the author of this work to attempt a natural
history of these animals, or their habitations, further than to select a
few of the most beautiful of the latter, in order to point out the method
of copying- them with accuracy.
Shells, like ssowers, have various forms and parts that can only be
described in technical terms, and therefore for the student who is un-
acquainted with conchology, or the natural history of shells, it will be
necessary briessy to explain them before they are used in the lesson.
Shells are divided by naturalists into two great families, called
univalves and bivalves.
Univalves (as the name denotes) have only one external covering;
such are the whelk, murex, or conch kind.
Bivalves are those that have two shells, which, joined together
by a hinge, consisting of certain teeth, are various in number and
form. This variety enables the naturalist to divide the different
genus of bivalves, for instance, the oyster from the muscle, and the
scallop from the cockle.
Mr. Perry observes, in his beautiful work on shells (a work which
every person should possess who desires to become acquainted with
conchology), that "the univalve and bivalve shells are found indis-
criminately scattered over the various coasts of the ocean, and the
largest are to be met with only in the torrid zone; but in the seas