(l) oli I'dln iiiihthiijia Siiilca 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 moUusks derive all their 

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

 deposited by some of those 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 deposit(.'(.l. If this is the case, we must allow that the 

 calcium is precipitated as carbonate l)y the C'U2 produced by the animal itself, together 

 with some other product to satisfy the SO4 ion. For the sea water does not contain a 

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

 of SO4 ions which must be taken care of. Steinmann's hypothesis of the formation of 

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

 wlinle matter is a problem for the physi(jlogical 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 aliility to deposit lime. ^loreovcr 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 Tridncnu shells of the Oreat Barrier reef, or the Hip'piLrik^ 

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

 sedentary members in which tlie .shell is enormously and grotescjuely distorted l)y exces- 

 sive lime deposition. Such is Rifhtluijeynin among the brachinpods, an organism originally 

 classed as a coral liecause of its remarkable form. Again, types such as tlie oyster, \vhich 

 are thin-shelled when very y(jurig (prodissoconch stage), become heavy-shelled l>y 

 abundant lime deposition after attachment, while the related Pcdm, whicli 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 bej^ond the normal stage required for the protection of 

 the individual. 



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

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

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

 sedentary life, and no give rise to genera and species normally of sedentary habit. In 

 this connection it is noteworthy that many moUusks will build excessively heavy sliells in 

 oM fige, and from this it might iie ;ngu(-d that types which in nonnal adulthood deposit 



