OSTREA. 



and adherent by its umbo in its young state, and some- 

 times, even until it has attained a considerable size; the 

 lower valve is smaller and rather flat. The hinge cannot 

 be said to have any teeth, though there is frequently near 

 to it a row of small denticulations, but these sometimes 

 extend all round the inner edge of both valves. The two 

 valves are united by a ligament, which as it lies between 

 the umbones, serves to separate them from each other 

 in a small degree, and can therefore be considered as only 

 partly external, though we know not how to say, that 

 it is in anywise internal, but it is not exposed when the 

 valves are closed : the disk or facet to which it is attached, 

 is generally of a subtriangular form; one of the points of 

 its triangle being close to the umbo, which point is rather 

 inclined to one side : this disk is tripartite, being divided 

 by two raised lines which divaricate from the umbo ; the 

 central portion is generally the largest and the deepest, but 

 the ligament is attached to all three : but as the shells in- 

 crease by age the older portions of the ligament, or those 

 nearest the umbo decay and do not serve any longer to 

 unite the two valves; consequently they leave between 

 them a small unoccupied space. There are two muscular 

 impressions in each valve, one of which is large, semi-orbi- 

 cular and nearly central ; the other is very small and placed 

 near to the hinge. 



The Genera with which Ostrea has been, and therefore 

 may still be confounded, are Crenatula^ Perna^ Malhus^ 

 Pedum^ Liimaj Pecten^ and Vulsella: from the two first, 

 Crenatula and Perna^ it may be distinguised by its hinge, 

 which in those Genera (if they are really distinct from, each 

 other) is composed of a number of longitudinal grooves 

 arranged side by side : from Malleus and Pedum^ by the 

 mode of its attachment, which in them is by a byssus, and 

 not by the outer part of the shell ; from Lima and Pecten^ 

 by its being irregular, as well as by the mode of its attach- 

 ment, for the shells of those Genera are regular and at- 

 tached by a byssus : and, lastly, from Vulsella^ by its mode 

 of attachment and by the hinge ; for though we believe the 

 Vulsellce are not actually attached, having never observed 

 any opening for the passage of a byssus, yet they generally 

 occur imbedded in sponges ; they differ also in the hinge, 

 the central portion in vulsella^ forming an internally pro* 

 jecting callosity. 



