46 SHELL GALLERY. 



edges, each being bordered by a short fringe, and the labial palps 

 are large and somewhat triangular. There are on each side a pair 

 of simple gills, which are closely striated ; the single adductor 

 muscle is large and nearly central. The Oyster is, except in the 

 very young state, entirely incapable of locomotion, and always 

 attached by the deeper valve to other shells, rocks, or other sub- 

 stances. The common British species is not full-grown until it is 

 about five or seven years old. A series of different ages, from the 

 " spat" to the adult form, is exhibited in Case 25 F. During the 

 months of May, June, and July the eggs are discharged into 

 the gills, where they remain until hatched ; and it is during 

 this period that oysters are " out of season." In the American 

 Oyster (0. virginica), on the contrary, the eggs are said to be 

 hatched outside the parent shell. Oysters of different kinds 

 are found on nearly every shore. The gigantic 0. gigas is said to 

 grow to the length of three feet in the Bay of Taichou, Japan 7 

 where it is commonly eaten. About two hundred fossil species 

 have already been described. 

 [Case The family of Anomiida contains a number of more or less pearly 



shells remarkable for a deep notch or hole in the lower or flat valve 

 through which a shelly plug passes, by means of which the animal 

 attaches itself to other shells, stones, &c. Anomia cenigmatica is 

 found adhering to leaves in mangrove-swamps. 

 rCase Th e Placunida, sometimes called Window-shells and Saddle- 



26, a- b.] Oysters, are very flat pearly shells with a remarkable hinge, which 

 consists of two long divergent teeth, like a A> to which the liga- 

 ment is attached. The species are few in number, and iuhabit 

 sandy shores of India, China, and North Australia. 

 rp In sections B-D of Case 26 is exhibited a number of shells which, 



26, b-g.] either from the attacks of worms or from reparation of injuries, or 

 from other causes are very much distorted in growth or monstrous. 

 In division E some sections of shells are exhibited showing their 

 internal structure. In the same and following division (F) are 

 some specimens of rocks, corals, and wood, illustrative of the per- 

 forating powers of the boring kinds of Mollusca ; and in division G 

 is exhibited a series of shells of pearl-forming mollusks. 



