Hyatt.] 406 [Nov. 16, 



acme of groups, are the functional relations of one class of mor- 

 phological modifications to those of another class and do not 

 properly include the morphological phenomena themselves or their 

 structural correlations. 



The necessity for a double set of terms may possibly not be at 

 first admitted by many zoologists on account of their too exclusive 

 devotion to the morphological side of their studies, but a very 

 slight experience in trying to express the serial correlations of 

 morphological and physiological phenomena will very soon show 

 them the convenience of such a nomenclature. Geologists have al- 

 ready arrived at this conclusion with regard to the classification of 

 strata in the earth's crust and have begun to use two parallel series 

 of terms one giving the nomenclature of the relations in time, 

 era, period, age, etc., and the other the faunal relations under 

 the headings of group, system, stage, and so on. 17 The time has 

 come for recognizing a similar parallelism between structural or 

 statical phenomena of organisms and their dynamical or physi- 

 ological relations in time, and it is necessary to separate these 

 clearly by different series of terms in order to see not only how 

 they are separable, but also their correlations. 



We have been more or less constantly observing and publishing 

 on the geratologic stages among fossil Cephalopoda for more 

 than twenty years and have repeatedly described the more or less 

 exact comparisons, which can be made between the different stages 

 of decline in the individual and the degraded forms occurring in 

 the same group. 



There were two stages in the old age period among Ammonoids : 

 the first of these can be designated as the Clinologic 18 stage. 

 This immediately succeeded the ephebolic period, and during its 

 continuance the nealogic and ephebolic characteristics underwent 

 retrogression. Ornaments, spines, and sutures degenerated and 

 lost their angularity, the ribs or pilse, and often the keel and chan- 

 nels, when the latter were present, became less prominent, and 

 before this stage closed the whorl itself sometimes decreased, show- 

 ing that degeneration in the growth force of the animal had taken 

 place. Similar phenomena can be easily observed in other de- 

 partments of the animal kingdom, notably in man, whose habits 

 tend to preserve life until he has attained extreme age. During 



17 G. K. Gilbert, Address, Am. Ass. Adv. Sci., 1887. 



18 KAiVto, to incline downwards. 



