Hyatt.] 824 [April 4, 



form of the Silurian Anarcestes, which we also recorded as occurr- 

 ing in nearly every group of the Goniatitinae and at the morpho- 

 logical base of the groups of the Clymeninae. This form of whorl 

 has almost disappeared in the Trias and its place is taken by quad- 

 ragonal derivatives, like Xenocliscus, and the compressed helmet 

 shape, which we have designated as the secondary larval ibrms. 

 Whatever form we may admit into the category of secondary lar- 

 val radicals, they must in all cases be cliscoidal, with open umbil- 

 ici ; and either themselves, or their in mediate ancestral forms 

 must be shells, without spines or prcminent outgrowths, thorgh 

 they may be costatecl, as in Sandbergeoceras, the immediate radi- 

 cal of the Prolecanitidae. The exact agreement between the 

 sutures and forms, and their development both in the series and 

 in the individual is not in the Trias substantiated by the observed 

 geological positions of many species, in fact the occurence of 

 Monophyllites, Sphingites, and Ceratites Sturii and the second- 

 ary radical, Dinarites Mohammedanus and other instances are 

 directly against these views. This, however, does not at all 

 alarm us, if there is any truth in the theory of descent, we feel 

 sure that the clue lies in the development of the individuals 

 which occupy the lower morphological borders of each series, anel 

 exhibit in their forms, sutures, and shells, the nearest approach to 

 the primary radicals of the Silurian or Cambrian. 



The Paleozoic or primary radical is Anarcestes, and its depressed 

 whorl becomes larval in the more involute forms of all the genetic 

 series with which we are acquainted among the Paleozoic Gonia- 

 titinae, being absent from the early stages only in forms with 

 gyroceran young, and it is inherited by all forms above the Car, 

 boniferous, at the earliest stage of the formation of the apex or 

 conch. 



The discoidal secondary radicals are unquestionably the nearest 

 allies of this primary radical, which occur in the Devonian, and 

 these in their turn have a similar relationship with the Dyassic 

 and Triassic species having similar cliscoidal forms and simpler 

 sutures than other more involute shells of their several groups* 

 We can, therefore, with the reservation, that the connections 

 have not been actually made, state that a great change takes 

 place in the Dyas, and that here or about the time of the end of 

 the Paleozoic the secondary larval radicals, or Mezosoic helmet- 



