20 



TYPE AMMONITES— VI April 



1926 



The genera in the Tables III — V are placed approximately in 

 chronological order, the latest above, the earliest below ; but in those 

 cases where several genera belong to the same date, their individual 

 order has no chronological signification. Of the others, the exact sequence 

 is not known for certain in one or two cases, like that of Kosmoceras : 

 its date may be a httle later, less likely any earlier. Otherwise, on the 

 whole, the evidence for the chronology of the genera is fairly clear. 

 They are distributed among South English strata in the following 

 manner : — 



Kosmoceratan North Oxford Clays 



Christian Malford and 

 Calvert Beds 



Reineckeian Almost no deposit 



Proplanulitan • • . • • • • Kellaways Rock 



Kellaways Clay 

 Macrocephalitan . . . . Upper Combrash 

 But the genera Kepplerites and Toricelliceras are unknown in England, 

 <ind may represent a time between the deposition of Upper Combrash 

 below and of Kellaways Clay above — a non-sequence in England. 



It is obvious from these Tables that the old familj- Kosmoceratidae 

 has become too unwieldy, and that some lines of separation should be 

 adopted. The following scheme is, therefore, proposed : 



Superfamily KOSMOCERATACEM, Nov. 

 Kosmoceratidae, as used in this work — see T.A. Ill, p. 53. 



I. Family GOWERICERATIDM, nov. 

 Genera which barely rise beyond the runcinate ventral stage, and drop 

 back, sometimes quite early, to rounded. Macrocephalitan and Prop- 

 lanulitan Ages. 



2. Family GULIELMICERATID^, Nov. 



Genera which reach ventral sulcation and ventral tuberculation, but 

 ■decline to smooth venter. Laterally bituberculate, with final preserva- 

 tion of inner marginal nodes. Proplanulitan and Kosmoceratan Ages. 



3. Family PARAPATOCERATID£, Nov. 



Genera of uncoiled forms — Criocones and Toxocones. Proplanulitan Age 



4. Family Kosmoceratidae, Hyatt 



This family name is now much restricted. 



Genera which reach ventral sulcation with strongly developed 

 bordering tuberculation, and considerable decline therefrom, but not 

 to smooth stage. Well-developed lateral tuberculation, with decline to 

 costation. Kosmoceratan Age [and into early Vertumniceratan]. 



The ventral tubercles are septate spines, which are frequently lost, 

 only the septate base remaining ; this base carrying the septum, or the 

 mark of the septum, is sometimes elongated to bridge two or more ribs — 

 it then resembles a clavus. 



The foregoing genera may be noted in more detail. 



Gowericeratidae. The genera of this family barely pass beyond 

 costate runcination, the venter is devoid of bordering tubercles, and 



