(i) 56 PaliKontologia Sinica Ser B. 



generally appear to stand on the ground that all the lime of the mollusk shell is furnished 

 by the animal, being derived from the food-supply (see especially Stempell 1900, and 

 Biedermann 1901), but it must be recognized that their generalizations are based on the 

 investigation of only a limited number of types. That marine mollusks derive all their 

 lime from the food, seems highly questionable when we consider the vast amount of lime 

 deposited by some of these organisms, especially sedentary types such as oysters, 

 hippurites etc. The conclusion seems unavoidable, that in some way the animal 

 appropriates lime from the sea water direct, or that in some manner the sea water gains 

 access to the region where lime is deposited. If this is the case, we must allow that the 

 calcium is precipitated as carbonate by the C02 produced by the animal itself, together 

 with some other product to satisfy the S04 ion. For the sea water does not contain a 

 sufficient quantity of COs ions ready to combine with the Ca ions and there is an excess 

 of S04 ions which must be taken care of. Steinmann's hypothesis of the formation of 

 ammonium carbonate, through normal decay of tissues, satisfies these requirements. The 

 whole matter is a problem for the physiological chemist, and its solution must be 

 left to him. 



One thing, however, seems certain, namely that in different organisms there is a 

 vast difference in the ability to deposit lime. Moreover in sedentary forms, lime 

 deposition is far more active than in free moving types, being least in planktonic types. 

 One need recall only the giant Tridacna shells of the Great Barrier reef, or the Hippurites 

 of the Cretaceous. Furthermore, other classes of organisms, normally thin-shelled, have 

 sedentary members in which the shell is enormously and grotesquely distorted by exces- 

 sive lime deposition. Such is Richthofenia among the brachiopods, an organism originally 

 classed as a coral because of its remarkable form. Again, types such as the oyster, which 

 are thin-shelled when very young (prodissoconch stage), become heavy-shelled by 

 abundant lime deposition after attachment, while the related Peden, which leads a free- 

 swimming existence, only builds a relatively thin shell. Any one who has seen the 

 ponderous oyster shells of the Tertiary, sometimes several inches thick, must agree that 

 lime deposition here has passed beyond the normal stage required for the protection of 

 the individual. 



Of course it may be argued that the nature and abundance of lime-secreting cells, 

 and their relative activity serves to determine the habitat of the organism. That, in 

 other words, types with a tendency towards excessive lime formation will assume a 

 sedentary life, and so give rise to genera and species normally of sedentary habit. In 

 this connection it is noteworthy that many mollusks will build excessively heavy shells in 

 old age, and from this it might be argued that types which in normal adulthood deposit 



