448 ME. S. S. BUCKMAN ON THE GROUPING OF SOME [Aug. 1 898, 



depressed-whorled, slowly-coiled, costate species, and certainly a 

 species with a simple suture-line not dependent with regard to its 

 inner lobes. Such would be the ancestor of EcMoceras^ a genus to 

 be regarded as one of the forerunners of the Hildoceratidse, which 

 attain to so much importance later ; and with that family it 

 should be classed. Certainly Echioceras is only a migrant from 

 some main stock existing in some unknown locality — perhaps a 

 pelagic series. It soon runs its course, but other migrants from this 

 stock appear at disconnected intervals daring this Age ; while after 

 its close the ammonite -faun a consists almost entirely of successive 

 migrant waves of Hildoceratidae, their similarity to each other 

 indicating a common origin. 



The hemera which succeeds that of raricostatum is dominated by 

 an entirely different series of ammonites — the genus Beroceras 

 of the family Deroceratidae, whose descendants, the Stepheoceratidse, 

 attain so much importance in the Stepheoceratidan Epoch. 



The two following hemerae saw the dominance of another family 

 — the Polymorphidae — in the genera Uptonia ^ and Cycloceras. 

 Possibly as belonging to the same family, or more likely to the 

 Deroceratidse, may be reckoned the dominant genus of the next 

 hemera — namely Liparoceras.^ Two more hemerae remain, and 

 these witness the incursion of yet another family, the Amaltheidse, 

 and the remarkably sudden disappearance of this branch of it with 

 the close of the hemera spinati. 



The Deroceratan Age, therefore, is not dominated throughout its 

 duration by any one family. Suitably, however, it begins with 

 the replacement of the Arietidae by the new series Echioceras,^ and 

 ends with the disappearance of the Amaltheidae to permit of the 

 next age commencing with the new incursions which dominate it. 



(c) The succeeding Age is the Harpocer a tan, consisting of ten 

 hemerae, and during nearly the whole time it is dominated by genera 

 of Hildoceratidae. During the earlier hemeree the genera Dactylio- 

 ceras and its allies — descendants of Deroceras — play a somewhat 

 important part; but then they become insignificant, and almost 

 disappear with the close of the hemera hifrontis,^ while successive 

 waves of Hildoceratidae continue to be dominant until the hemera 

 dispansi. At the close of this the Hildoceratidae practically dis- 

 appear for a time, and their place is taken by Dumoriieria, a genus 



^ See p. 453. 



^ There may be more than one series in Liparoceras ; they may be homoeo- 

 morphs, descendants of genera belonging to the two families. 



^ It may be noticed that Haug divides the Oharraouthian (strata of Dero- 

 ceratan Age) from the Sinemurian (strata of Asteroceratan Age) at the base of 

 iheraricostatum-zone, but for a totally different reason — astratigrapbical one — 

 namely that ' la trangression du Lias moyen parait comraencer avec la zone a 

 Caloceras raricostatum.^ (Article 'Jurassique' in 'La Grande Encyclopedic,' 

 Paris, 1894, p. 326.) 



* The idea that Jbactylioceras is the forerunner of Perisphinctes cannot be too 

 strongly condemned — as a study of the suture-lines is sufficient to show, or 

 better srill, a study of the inner whorls of Perisphinctes. That genus is the 

 ■non-tuberculate degenerative of Stepheoceras (Stephanoceras). 



