168 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



varieties of Ostrese in which the umbones have been involutely pressed 

 over ; and those in which the umbones are distorted to the left have 

 been set apart by Say under the new generic title of Exogyra. The first 

 of these genera has been the most esteemed by conchologists, because 

 the distinction is tolerably well marked, and particularly refers to a large 

 number of fossil species ; we conceive, however, that the legitimate rank 

 of this division will be fully maintained by regarding it merely as a section 

 of the primitive genus (gryphoid Ostrecs, e. g.). These capricious altera- 

 tions are highly injurious to the true principles of classification ; instead 

 of simplifying the arrangement, they tend to confuse it ; and we cannot 

 but deprecate the innovations of those authors, who attempt to give a 

 place in the nomenclature to every little variation in the inscrutable ope- 

 rations of Nature. 



The shell of Ostrea may be described as being attached, inequivalve, 

 and irregular ; the umbones are very unequal, somewhat divaricating, 

 and receding from each other as the area between them becomes enlarged 

 by the increase of the shell ; the valves are fibro-laminar, and sometimes 

 foliated ; the lower valve, which is always the larger, is concave, and the 

 upper or smaller valve is generally flattish. The hinge is destitute of 

 teeth, and the ligament is either internal or semi-internal, attached to the 

 umbonal area. The muscular impression is situated near the centre, and 

 the mark of a minute accessory cartilage has been noticed beneath the 

 hinge, analogous to that of the Pinnee. 



We refer our readers to the various Encyclopaedias for accounts of the 

 manner in which immense beds of the Ostrea edulis are cultivated for the 

 purposes of food. This species has long been a common article of luxury 

 throughout the whole of Europe ; it was in general consumption in the 

 time of the ancient Romans, and the ompeiov of the Greeks is frequently 

 mentioned by the old writers*. 



* An interesting account of the oyster-beds of America has been very recently published 

 by Dr. Augustus Gould of Boston, in his ' Report on the Invertebrate Animals of Massachu- 

 setts' (page 358). It does not come within the object of the present work to enlarge upon the 

 habits of the Mollusca, or we should certainly have given some extracts from this elaborate 

 report. 



