﻿37G 



When we attempt to understand these pathologic uncoiled series 

 and forms, which show by their close-coiled young that they were 

 descended from close-coiled shells, we find ourselves without com- 

 parisons or standards in the early life of the individual. The laws 

 of geratology — that the old age of the individual shows degradation 

 in the same direction as, and with similar changes to those which 

 take place in successive species or groups of any affiliated series of 

 uncoiled and degraded forms — here come into use, and serve to ex- 

 plain the phenomena. This correspondence is shown in the uncoil- 

 ing of the whorls, loss of size, the succession in which the orna- 

 ments and parts are resorbed or lost, the approximations of the 

 septa, and position of the siphuncle. It is quite' true, as first stated 

 by Quenstedt and also by D'Orbigny, that every shell, when out- 

 grown, shows its approaching death in the closer approximation of 

 the last sutures, the smoothness of the shell, the decrease in size, 

 etc.; but, in order to realize that these transformations mean the 

 same thing as those which take place in any series of truly retro- 

 gressive forms, we have to return to the types in which unfavorable 

 surroundings have produced distortions or effects akin to what 

 physicians would term pathological. 



This frequently happens in small series of Nautiloidea ; and, if 

 we confine ourselves to these, we can make very accurate com- 

 parisons : or, on the other hand, in the case of the Ammonoidea, 

 we may trace the death of an entire order, and show that it takes 

 place in accordance with the laws of geratology. Such series, 

 among the Nautiloidea, are abundant in the earlier formations ; but 

 they have not the general significance of the similar forms among 

 the Ammonoidea, and can be neglected in this article. There are 

 no known cases of degraded series of uncoiled forms among the 

 ammonoids of the earlier or Paleozoic periods ; they may have oc- 

 curred, but they must have been excessively rare. 



In the Trias and early Jura, pathologic uncoiled forms are rare 

 among ammonoids, but in the Middle and Upper Jura thev increase 

 largely ; and finally, in the Upper Cretaceous they outnumber the 

 normal involute shells, and the whole order ceases to exist. Neu- 

 mayer has shown, that a similar degradation occurs in all of the 

 normal ammonoids of the Cretaceous, and that their sutures are less 

 complicated than those of their immediate ancestors in the Jura, 

 This proves conclusively, that the degeneration was general, and 

 affected all forms of Ammonoidea at this time ; since the uncoiled 



