216 Mr. Gray's Conchological Observations. 



a peculiar theoretical doctrine, that the shell was similar to the 

 Rete Mucosura of animals, instead of being analogous to the hair, 

 nails, and other appendages of the skin, proposed the name of 

 Drop Marine ; but as this web is found in land and fresh-water 

 shells, as well as in marine ones, this name is evidently worse than 

 the former. Now although I allow that the shells of these various 

 stations, have their peculiar kind of epidermis, I cannot think 

 that it would be well to give three names to the same sort of 

 covering ; to obviate this difficulty, I propose to call this web 

 JPeriostraca, a name formed after that of the membrane which 

 invests the bones of quadrupeds, which is called Periosteum^ and 

 which surrounds the bone as the Periostraca surrounds the shell. 

 This name carries with it no theoretical view. 



In my observations on the formation of shells, I stated that they 

 were formed by a successive deposition of layers of shelly matter; 

 now each of these layers consists of a layer of animal matter, and 

 a certain quantity of carbonate of lime, according to the structure 

 of the shell ; these layers may mostly be traced by placing the 

 shell in very dilute muriatic acid, so as very slowly to dissolve the' 

 calcareous matter, when the animal matter will be left in the 

 form of plates, giving in fact the skeleton of the animal fabric of 

 the shell. The same circumstance may be shewn by exposing 

 the shell to the influence of fire, when the animal matter is des- 

 troyed, and the calcareous alone left ; but if this latter process 

 is not carried to too great an extent, the plates of animal matter 

 may be seen charred, forming black lines in the white chalky 

 matter, if the shell is broken transversely. 



Now from attentive examination I am inclined to believe, that 

 the Periostraca is formed by the outer edges of the plates of 

 animal matter, [which has no cretaceous matter deposited in it, 

 or only a very small quantity,] being soldered together and 

 forming a kind of external coat. I am more inclined to this opin- 

 ion from the well known fact, that the innermost part of each of 

 the plates of shelly matter, is much the most solid, and contains 

 the greatest portion of earthy matter ; from this it is often called, 

 when reflected over the back of the shell, as in the Cypra^a;, the 

 Porcellaneous <;oat; and we find as we come toward the outer 



