Nol I. Grabau — Ordnvician Fossils from North China (i) o7 



much lime, belong to phylogerontic series. Why senile individuals and senile races (if 

 such they are) should have their lime-secreting mantle cells over-stimulated so that they 

 deposit an excess of lime, is not quite clear. Nor is it easy to understand why they 

 should absorb more hme salts from the sea or from the food (if that is the sole source of 

 the lime salts, which is very doubtful) in old age than in their younger stages, especially 

 as there is often no corresponding increase in size of the shell-building mantle. If on the 

 other hand, deposition of lime is more or less a purely chemical process, as Steinmann 

 holds, and that its rate and amount of formation depends upon the rate of production of 

 reagents which precipitate the salts, either from the normal secretion of the animal, or 

 from sea water, which in some way (by osmosis?) has gained access to the regions of 

 deposition, then we can understand that with increasing old age, or increasing senescence 

 of the race, increased decay of organic cells Ijrings with it the increased production of 

 ammonium carbonate, with the result that lime deposition also becomes augmented. 



The fact that lime is not deposited upon the periostracum, which both Stempell 

 and Biedermann cite as ample refutation of Steinmann 's theory, can in reality not be 

 regarded as such, for the completed priostracum, though of conchyolin, has essentially 

 the character of an inorganic body, and does not produce the necessary reagents. 



I am not advocating the direct precipitation theory of Steinmann, but it appears 

 to me that the pure secretion theory, which refers lime deposition in molluscan shells 

 solely to the epithelial cells of the mantle, or to special lime-secreting glands, meets with 

 great difficulties when it is invoked for the explanation of excessive lime deposits, 

 especially if all the lime salts are regarded as derived from the food of the animal and 

 none from the sea water direct. 



If in a cephalopod shell, the processes which make for lime deposition are most 

 active at the growing edge of the mantle, the shell is rapidly elongated, while, by the rest 

 of the mantle surface, only a thin nacreous shell-layer is formed. If the growth at the 

 mantle edge is so rapid that the length of the shell eventually exceeds the stretching 

 power of the animal, a periodic forward movement of the -whole animal in the shell takes 

 place, whereupon the continued separation of lime over the now free basal portion of the 

 mantle-enclosed body, results in the formation of a septum. If the lime-separating 

 processes are uniform all over the mantle surface of a cephalopod, (as they are in oysters 

 among pelecypods), the basal part of a tubular or conical shell, such as an orthoceracone, 

 will be filled solidly by successive layers of lime. These may have the form of consecutive 

 endosheaths, or of crystalline lime with definite layers marking resting stages at inter- 

 vals, i. e. of successive distant endosheaths with crystalline lime-filling or stereoplasm 



