﻿232 INFERIOR OOLITE AMMONITES. 



One of the most interesting points in connection with the genus Dumortieria 

 is what may be called the " archaic " character of the " less-developed " shells. 

 The more or less rounded whorls, the straight ribs, and the slightly-carinate 

 ventral area take us back to the much earlier (Lower-Lias) forms of Ophioceras ; l 

 and Hyatt rightly enough appreciated this when he placed Am. Levesquei in the 

 genus Ophioceras. 2 



The genera Dumortieria and Grammoceras have, superficially, a very great 

 resemblance ; but their descent is certainly distinct. This, however, is a matter 

 which can only be learnt by the examination of a large series of species. When 

 dealing with single specimens, and wishing to determine their genus, the reader 

 should consult page 162, and remember that the characteristics of Dumortieria are 

 ribs straight on the lateral area and very little projected ventrally, long siphonal 

 and superior lateral lobes, the inner portion of the suture-line brought down 

 obliquely (dependent). 3 



The origin of the genus Dumortieria may be traced to a smooth species, say 

 Agassiceras miserabile* in the Obtusum-zone. This species gave rise to the smooth 

 Polymorphites polymorphous, which passed into Pol. polymorphic lineatus with fine 

 ribs, and into Pol. polymorphous costatus with stronger and more distant ribs. 

 This development, carried on in the same manner, would produce Gatulloceras 

 Vernosse (Zittel), a species with well-marked ribs but no carina. Now, this species 

 occurs in the MargarUatus-zone ; and it should be noticed that its development 

 from the smooth Pol. polymorphic is exactly parallel to the changes which, in the 

 Lower Lias, produced such species as Am. carusense, d'Orbigny, or Am. planicosta, 

 Sowerby, from a smooth ancestral form. 



Between Gatulloceras Vernosse of the Middle Lias, and the species of Catullo- 

 ceras and Dumortieria of the Jurense-zone, is a break, during which no species of 

 the genus can be recorded ; and, therefore, the changes necessary to produce the 

 Jurense-zone species from the Middle-Liassic ancestor must be matters of hypothesis. 

 The great similarity between Dumortieria prisca and Gatulloceras Dumortieri 

 favours the view of their derivation from a common, and not very remote, source. 

 The addition of a carina on the ventral area cutting across the ribs would convert 

 Gatulloceras Vernosse into Gatull. Dumortieri. To evolve Dum. prisca, however, 

 more changes must have taken place. First, the ribs of Gatull. Vernosse are 

 coarser and more distant on the outer than on the inner whorls, and the supposed 



1 See Aeg. liassicum, intermedium, Belcheri, figured by Dr. Wright, ' Lias Amm., Pal. Soc.,' pi. xv. 

 These species are now described as Caloceras ; Hyatt, " Genesis of Arietidse," ' Smithsonian Contribu- 

 tions to Knowledge ' (4to.), vol. xxvi, p. 141, 1889. 



2 Hyatt, ' Bull. Mus. Comp. Zool.,' No. 5, p. 93. 



3 At p. 162, line 23 from top, " practically no inferior lateral lobe " is not quite correct. It is 

 better to say " small inferior lateral lobe." 



4 See, however, Haug, " Polymorphidse," ' Neues Juhrbuch fur Mineral.,' &c, Bd. ii, 1887. 



