450 MR. S. S. BIJCKMAN ON THE GROUPING OF SOME [Aug. 1 898, 



ceratidse takes place. It is true that this disappearance is very 

 important, because with the disappearance of the Hildocerafcidse 

 ends the dominance of that type of ammonite known as Arietidae 

 and Hildoceratidae. Then with a short period of dominance of the 

 Sonnininse there comes the dominance, at any rate for the remainder 

 of * Jurassic ' time, of an entirely different series — a very much 

 more elaborately-developed ammonite — a descendant of the Dero- 

 ceratidae, which several times endeavoured to become dominant 

 before — namely, the Stepheoceratidae. However, in the discitce 

 hemera the Sonnininae become dominant, so that both in individual 

 size and in number of species they entirely overshadow the last 

 remnants of the Hildoceratidae. From a zoological point of view 

 the Sonnininae are the most important ; and on this ground it is now 

 proposed that the Ludwigian Age and the Arietidan Period should 

 terminate with the close of the hemera concavi. 



The Stepheoceratidan Epoch commences with the Sonninian 

 Age (e), comprising five hemerae during which the Sonnininae are 

 in their acme and their paracme. In the first four hemerse they are 

 certainly dominant ; in the last they are few in numbers and small 

 in size, and the importance of the Stepheoceratidae which have 

 accompanied them throughout is at last firmly established. The 

 Sonninian Age is fittingly brought to a close with the final 

 disappearance of the Sonnininae in the hemera Blagdeni. 



The Parkinsonian Age (/) commences with the first appear- 

 ance of those peripherally-sulcate Stepheoceratidae, hitherto called 

 Cosmoceras, and now Cosmoceras and Parhinsonia, which play so 

 important a part in the succeeding hemerae. But, as to the suitable 

 subdivisions of Parkinsonian time, we are in this country under 

 difficulties, because of the non-ammonitiferous nature of the deposits, 

 Porthis reason I have inserted in Tables I & II two names of brachio- 

 poda as hemeral designators, but they are only intended for use until 

 it can be certainly shown during what ammonite-hemera they lived. 

 As to the time when the Parkinsonian Age should close, I do not 

 feel competent to express an opinion : it must depend entirely on 

 the ammonite-evidence, and be regulated by the time of disappear- 

 ance of some portion of the essentially Parkinsonian fauna, or the 

 incoming of a new fauna. 



It may be remarked, however, that from the Sonninian Age 

 onwards it would appear that all the ammonite-fauna, excluding 

 the Lytoceratacese, consists of two main families, the Stepheocera- 

 tidae and the Oppelidae. Developments of certain genera of these 

 families may attain sufficient importance to be given family rank, 

 but that will not alter the facts of the case, that after the Sonninian 

 Age there are found no more members of the families Arietidae, 

 Hildoceratidae,^ Polymorphidae, nor apparently of the Amaltheidae. 



1 The most Hildoceratoid genera of the Stepheoceratidan Epoch — namely 

 Hecticoceras and Lunuloceras — are placed by Bonarelli in the family Oppelidae. 

 This is most likely the true reading of their affinities; see Boll. Soc Malac. 

 Ital. vol. xviii, p. 77. 



