50 



monly known as Saddle Oysters, from their peculiar saddle-shaped bend, 

 and Chinese Window Oysters, from the circumstance of their being used by 

 the poorer inhabitants of the Celestial Empire for glazing windows. The 

 valves press so closely one upon the other that it would seem scarcely pos- 

 sible for the animal to be able to close its shell, and until a living speci- 

 men falls into the hands of a malacologist skilful enough to observe it, the 

 habits of Placuna in this respect must remain a mystery. 



The converging tooth-like ridges of the hinge are a conspicuous feature 

 of the genus ; and the ligament rests partly on these and partly along the 

 hinge margin on either side of the umboes. 



Species. 



1. Lincolnii, Gray. 3. placenta, Linn. 4. sella, Gmelin. 



2. papyracea, Lam. 



Figure. 



Placuna placenta. PI. 27. Pig. 158. Shell, of a small specimen, with 

 the upper valve removed, to show the converging ridges of the hinge. 



Genus 4. OSTREA, Linn mis. 



Animal ; edges of the mantle dot/Me, and each bordered by short 

 tentacular fringes ; branchial leaflets not doubled on themselves ; 

 labial appendages triangular, connected around the mouth by a 

 plain membrane ; no foot. 



Shell ; affixed, inequivalve, irregular, with the umboes mostly very 

 unequal and divaricate, receding from each other with the 

 growth of the shell ; lower valve the larger, convex ; upper 

 valve generally rather flat ; hinge toothless, with the ligament, 

 either internal or semi-internal, attached to the umbonal area. 



The renowned mollusks of this genus produce shells of such irregular 

 growth that nothing short of a baronetcy, it has been said, will ever in- 

 duce a conchological commoner to monograph them. Their habit of getting 

 into nooks and crevices when young, though in their metamorphic destiny 

 denied the power of escape, obliges them to build their shell in all sorts of 

 improbable places, and the result is a dislocation of forms and characters 



