62 ZOOLOGY. 



Lranches, wliere they may be seen suspended at low tide.* There are four 

 lanceolate labial palpi ; and the branchia?, two upon each side, are 

 conspicuous objects when the shell is opened. Tlie very large liver is 

 recognised by its dark color, which causes it to be indistinctly seen through 

 the translucent integuments. There is no appearance of a rudimentary foot, 

 but eyes have been detected. 



The ancient Romans were very fond of oysters, and soon discovered 

 those of Britain to be superior to their own, and imported them in winter 

 packed in snow, aud in such a manner as to prevent the valves from 

 opening, a mode still practised when oysters are to be transported a 

 considerable distance. Previous to this period Sergius Aurata had invented 

 and practised the art of breeding oysters in artificial beds, turning his art to 

 great profit. This is still practised in the Mediterranean, where ponds arc 

 used, into whicli the sea enters at liigh tide. About Naples oysters attach 

 themselves to sticks, and here numerous poles are stuck into the bottom, 

 and when the oysters which become aflixed to them have acquired a 

 sufficient size to be taken, they are collected by withdrawing the poles. 

 Oysters attain a marketable size in four or five years, although not yet fully 

 grown, large individuals being considered less delicate than those of a 

 medium size. Those are most highly esteemed which have grown in the 

 mouths of rivers, where the water is less saline, and it is probable that their 

 food varies according to the locality. Oysters feed upon infusoria ; and 

 when certain green kinds are abundant, they impai't a green color to the 

 animal, a color which is often incorrectly attributed to some mineral 

 substance. The extent to which oysters ^rc consumed would exterminate 

 them in accessible localities were they not prolific. A single oyster 

 may eject 50,000 or G0,000 eggs in a year, commencing in the spring and 

 continuing through the summer. 



Deshayes does not think tlie genera Gryplicva^ Lamarck, and Exogyra., 

 Say, are sufficiently distinct from Ostrea ; whilst Yon Buch, the celebrated 

 geologist, insists that they are founded upon good characters. The former 

 author is inclined to look for variations in the soft parts of allied genera, 

 although in such the shell usually presents the more prominent generic and 

 specific characters. Variations in the shell are as important among the 

 Mollusca as in the teeth and horns of the Mammalia, or the bill and ft^hers 

 of birds ; and, although a species may be found which associates it witli two 

 genera, this is not sufficient evidence that the two should be united. 

 According to Deshayes, Ostrea passes by insensible gradations into 

 Exogyra, and this, by equally gradual steps, returns to Ostrea, so that in 

 his opinion the species of the three form but one natural genus. 



Fam. 4. PectinidcB. In Peden {pi. Y6, figs. 27 to 29), the chief genus 

 of the fomily, the shell is regular, toothless, inequivalve, eared at the hinge 

 margin ; ligament entirely internal, and placed in a triangular depression ; 

 surface often covered with ribs. Mouth Avith deeply cut lips and a pair of 

 palpi on each side; mantle disunited, margin with numerous cilia, having 

 eyes betM'een them ; branchiae sub-divided into separate parallel filaments ; 

 ibot small and dilated, a byssus sometimes present. The genus Pecten is 

 26G 



