WHAT IS AN OYSTER ? 65 



that testaceous molluscs are animals with whom ossification 

 is thrown out on the external surface in place of the 

 interior, as in the mammals, birds, reptiles, and fishes. 

 In the case of the superior animals the bones lie in the 

 depths of the body ; in the shelled Mollusca the bones are 

 placed on the superficies. It is the same system reversed." 

 Other Zoologists reject as altogether untenable both these 

 comparisons. 



"The shell which serves as a dwelling and a shelter 

 cannot," say these authors, " be considered as a skeleton, 

 because it does not assume the external form of the animal ; 

 because it does not attach itself to the organs of locomotion ; 

 and, finally, because it is the product of secretion, which 

 increases in proportion to the development of the body 

 itself." (a} The last two reasons appear to me to be the 

 most acceptable. 



The upper shell is flat, thinner than the other, and 

 marked by tolerably regular crescentic lines ; the under 

 shell is convex externally and concave internally, its outer 

 surface is rugged, with bold transverse marks of growth, 

 longitudinal ridges, and intervening furrows. It is much 

 stouter than the flat shell, and is sometimes very thick and 

 coarse. 



This theory of the " upper" and " under' shell of the 

 oyster, or the "right" and "left" valve belonging to it, 

 has been strongly debated by naturalists, and in spite of 

 seeming agreement amongst them at the present time 

 relative thereto, the dispute crops up now and then only to 

 come to an unsettled conclusion. 



" Indeed, there have been so many controversies about 

 the natural history of this animal as to render it impossible 

 to narrate in the brief space I can devote to it a tenth part 

 (a) " The Ocean World," p. 316-17. 



