1887.1 403 [Hyatt. 



radical Endoceratidae. Microsiphonulation became silphologic in 

 the Orthoceratidae, and the smooth shell which they evolved was 

 subsequently inherited among Nautiloids, Ammonoids and Belem- 

 noids during the younger stages in all the species of these orders. 

 Other forms, with depressed and involved whorls, were introduced 

 in the main stock of radicals among Goniatitinae, and were 

 modifications of the smooth cylinder of the simpler Orthoceratidae 

 with its microsiphon. These in turn became the proximal radi- 

 cals of derivative groups. Thus the Anarcestes 13 among Goniati- 

 tinae became the radicals of the Ammonoidea, and the smooth 

 silphologic stages of all Ammonoids after the expiration of the 

 Devonian were like the adults of these lowest forms of Goniati- 

 tinae. This later acquired silphologic stage has therefore been 

 styled the Goniatitinula. 



It has also been found that, in tracing the descent of forms with- 

 in smaller groups, sub-orders, families, and genera, it is practi- 

 cable, as in the case of the family of Endoceratidae, to prove that 

 characteristics usually appear first in adult stages and are then 

 inherited at earlier and earlier stages in successive species of the 

 same stock, whether they occur on the same horizon, or in differ- 

 ent horizons. The adolescent or Nealogic 14 stages are of as great 

 importance for tracing the genealogy of small groups as are the 

 silphologic characters in larger groups. Thus one can speak in 

 definite terms of the relations of the nealogic stages, and their 

 meaning, and importance in tracing the genealogy of families and 

 genera, without danger of confusing them with the characters of 

 any of the silphologic stages. 



After the silphologic and nealogic stages have been disposed of 

 there still remains the adult period, which is equally important in 

 genealogical investigations, since it enables the observer to study 

 the origin of many characters, which afterwards become silphologic 

 and nealogic in descendent forms. 



It is not uncommonly assumed, that adaptive characters ap- 

 pearing in embryos and larvae are apt to be transient and have 

 but little effect on the subsequent histor}' of the early stages in 

 the same group ; also, that such characters have appeared just as 

 readily in the larvae as in adults. Up to the present time this has 



m Gen. Ceph. Proc. Bost. Soc. Nat. Hist., xxn, 1880, p. 305. 

 14 jVeaA>Js, youthfulness. 



