53 Observations on the Petri/actiun of Shells 



difference is so great, that if the carbonate of crystalline lime had 

 not preserved the general form of the shell, it wonld be impossible to 

 recognise the origin of their now bodies. We have also found a Tri- 

 ton, the modifcrum of Lamarck, which presented singular circum- 

 stances. All the inequalities found on the surface in a fresh state 

 liave completely disappeared, and it has become quite smooth ; it is 

 wholly transformed into a crystalline limestone. One of its sides 

 has lost a part of its substance; and this opening seems to have 

 served as a passage to the lapideous liquids which have penetrated 

 into the interior of its cavity, and by accumulating there, have given 

 it a very great density and hardness. We likewise possess a curious 

 specimen of a mass of shells petrified and agglutinated together, 

 found in the neighbourhood of Algiers. We can distinguish among 

 these shells, 3Iurex tranaclus and araitinus, Natica cruentata, 

 Venus verrxicosa and gassina, Cardium tuberculatum and eduli, 

 Pectcn glaber, and Pectuncidus glycimeris. We likewise notice a 

 Lucina, not sufficiently entire to be determined; and, lastly, a cast 

 resembling Blytilus afer, which is known to live on the coasts of 

 Barbary. Among the most remarkable individuals of the last men- 

 tioned localities, and which, like these just spoken of, have been trans- 

 formed into crystalline alabaster, we may mention particularly a 

 Triton modifcrum of large size ; it is almost double the dimensions 

 of those belonging to our coasts ; numerous invei'se crystals of car- 

 bonate of lime are deposited in its interior. We likewise possess ano- 

 ther fi-agment about 40 square centimeters, composed almost entirely of 

 the ngglutinated valves of Petcunculi, which are likewise transformed 

 into crystalline alabaster. The valves are soldered to each other by 

 a gluten of the same nature, the whole as solid and brilliant as that 

 composing the valves themselves. This specimen, the origin of which 

 is vmknown to us, appears, however, to come from our own coasts, 

 for it existed in the collection of the Faculty of Sciences, long before 

 we had formed settlements in Aloiers. 



All the shells in the bosom of seas do not undergo the same kind 

 of alterations. For the most part, Oysters and Pectens receive the 

 lapidific fluids only between the leaves of their lamina3, which renders 

 them more solid and stony than in their fresh state. Often the 

 species of this genus not having very thick valves, such, for example, 

 as Ostrea cristata, are impregnated with a calcareous gluten, which 

 solders the valves together, in the same manner that takes place in a 

 great number of fossil species, particularly those of the secondary 

 formations. Sometimes the stony valves of large oysters, particu- 

 larly those of Oxtrea edidis in the Mediterranean, are covered on the 

 outside with crystals of the carbonate of lime. When this quantity 

 is considerable, it renders these oysters as dense as those of geolo- 

 gical times. The superior valves of the Pectens, are likewise covered 

 with small calcareous or sandy deposits ; but the latter are never 

 abundant, the valves of these shells not being of sufficient thick- 

 ness. 



