1887.] 407 THyatt. 



this period there is a steady loss of the differential characters ac- 

 quired during the stages of progressive growth and there is a 

 tendency to resume the proportions and aspect of the earlier 

 nealogic stages. In man, baldness of the head, loss of teeth and 

 resorption of the alveoli, loss of the calves, rotund stomach, and the 

 return of early mental peculiarities, are phenomena of similar im- 

 port. 



The last changes in the ontology of the animal may be termed 

 the Nostologic stage, 19 and during this stage these tendencies 

 reached their highest expression. Among Ammonoids the orna- 

 ments were all lost by resorption, the whorl became almost as 

 round and smooth as it was in the silphologic stage, and in ex- 

 treme cases it was separated from the next whorl, leaving a per- 

 ceptible gap. This almost complete reversion to the aspect of the 

 silphologic stage can of course only occur in animals which attain 

 an extreme age. 



The correlations of Clinology are exact, and indicate the changes 

 which may be expected to occur in the same group whenever de- 

 graded or aberrant species can be traced in a more or less con- 

 tinuous series of graded modifications starting with any given 

 normal form. Many such series have been traced, and these are 

 recognized now by all paleontologists as genetically connected. 

 They began with normal, close coiled, ornamented shells, the de- 

 scendants were smaller, showing a tendency to be less involved by 

 growth, to lose their ornaments, and simplify the outlines of the 

 sutures, though they had coiled young stages similar to those of 

 the normal forms from which they must have originated. 



The correlations of Nostology can only be artificially separated 

 from those of Clinology, but there existed one class of forms which 

 can be compared only with the nostologic stage. These are the 

 degenerate straight baculites-like shells, which belong to several 

 distinct genetic series and should often be widely separated on 

 that account. Their resemblances are undoubtedly close, but they 

 are due to degeneration and, therefore, simply homoplastic. Nat- 

 uralists sooner or later will begin to recognize that degeneration 

 may produce close representation in forms having distinct origins. 

 The Baculites is a smooth, straight, cylindrical though slightly 

 compressed shell, which has so completely reverted that it resem- 

 bles an Orthoceras, though it is an unquestionable Ammonoid of 

 the Jura and Cretaceous. 



19 Noo-to?, a return. 



