26 TYPE AMMONITES— VI Aug. 



they are not congeneric : the style of ribbing is different — the point of 

 bifurcation much nearer the periphery in Ap. decipiens than in the 

 other species. 



This introduces a question of chronology which is intimately bound 

 up with a question of the systematic position of species. Faunal 

 similarity of two deposits well separated in point of time has misled 

 observers into saying that the two deposits are isochronous. This is 

 an important point, about which a separate statement must be made 

 below. 



The date of Dr. Neaverson's memoir : In p. 24 above I have 

 given a query as to the date of this publication. Dr. Neaverson has 

 since kindly written to say that " the exact date of publication is 22nd 

 Dec, 1925." 



The species called pseudogigas : The propriety of selecting as a 

 neotype of Blake's Ammonites pseudogigas a specimen from the Creamy 

 Limestones of Buckinghamshire has been questioned. It seems advisable 

 to make the position clear. In the stratigraphical part of his paper 

 on the Portland Rocks (Quart. Journ. Geol. Soc, xxxvi, 1880), Blake 

 cites the species from 3 or 4 horizons : — i. Shell Bed of Portland (p. 192) ; 

 2, Creamy Limestones of Bucks (p. 216) ; 3, Rubbly Beds of Bucks 

 (p. 218) ; 4, Shotover and glauconitic sands (p. 225). No. i is of 

 Behemothan Age, No. 2 Gigantitan, No. 3 Behemothan, but not, 

 perhaps, isochronous with No. i ; while No. 4 is Paravirgatitan of my 

 Chronological Scheme, T.A. Ill, 26. It is obvious that the name was 

 quite uncritically applied to several different species. Blake's types 

 cannot, it seems, be identified, so the selection of a neotype is governed 

 by the following principle — a specimen from one of Blake's cited locahties 

 and horizons agreeing with the details which he gives in his palaeonto- 

 logical description (p. 228). There he says : 



" Ammonites pseudogigas, spec. nov. I confer this name on 

 " certain specimens which have in the inner half of the whorl 

 " large knobby ribs which bi- and trifurcate. It has the whorl 

 " as much inflated as in A. gigas ; but the ribs are more 

 " numerous than the knobs in the latter species, and are 

 " more truly ribs." 

 The statement about the inflation settles the species to be chosen — 

 Am. gigas at a diameter of 221 mm. has an inflation of 47 per cent. ; 

 the species from the Creamy Limestone figured by me as neotype of 

 Am. pseudogigas has at 215 mm. an inflation of 48 per cent. My 

 experience of Portlandian Ammonites shows that the species from the 

 Creamy Limestones of Bucks must be the one from which Blake took 

 this detail of the inflation, and that no other species from other Port- 

 landian beds attains such thickness and has at the same time the stout 

 ribs which Blake mentions. 



Therefore, Ammonites pseudogigas, or, as it is now, Trophoniies 

 pseudogigas, cannot be quoted as a species of the lower portion of the 

 Portlandian stone-beds (Behemothan Age) ; but it is a species of the 

 upper portion of these stone-beds (Gigantitan Age) just earlier than 

 Gigantites. The species which may take the place of pseudogigas as a 

 zone-fossil for a portion of the lower stone-beds is Kerberites kerberus, 

 possibly not one of the species which Blake called pseudogigas. 



