﻿DUMORTIERIA. 235 



the ventral area, there are in the Dumortieria-ancestry the following changes : 

 smooth, ribbed, carinate ; but in the Grammoceras-ancestry , smooth, ribbed, cari- 

 nate, carinate and sulcate (Arietan), degenerating into carinate. 



If anyone trace the gradual mutations expressed by Dum. prisca — Levesquei 

 striatulocostata — radians — Moorei, and compare them with the mutations Gramm. 

 toarcense — striatulum — mactra, he will find that the species of each line run almost 

 insensibly one into another in the order stated. Further, he will find that though 

 the earlier forms of the two genera — Dum. prisca and Levesquei and Gramm. toar- 

 cense — are very dissimilar, yet the later forms, Dum. Moorei and Gramm. mactra, 

 are almost indistinguishable. It is very important to bear in mind the differences 

 between the earlier species : first, the total absence of hereditary sulci — or of that 

 flattened ventral area betokening the former existence of the same — in Dam. prisca 

 and Dum. Levesquei, and the presence of this character in Grammoceras toarcense 

 and so many other species of that genus ; secondly, the suture-line — and no better 

 comparison can be made than between the sutures of Dum. Levesquei (PI. XXXVII, 

 fig. 5) and Gramm. toarcense (PL XXVIII, fig. 6). It is by the loss of ventral 

 furrows that Grammoceras converges to Dumortieria ;* while it is by the gradual 

 modifications or reduction of the salient points of its suture-line 2 — the long 

 siphonal and inferior lateral lobes, deep siphonal and superior lateral saddles, 

 dependent inferior lateral and auxiliary lobes — that Dumortieria converges to 

 Grammoceras. Further, Dumortieria converts a rounded, little-carinate ventral 

 area into an acute carinate area ; Grammoceras converts a carinate and sulcate 

 ventral area into an acute carinate area; and so convergence is accentuated. 



The geological position of the genera Dumortieria and Gatutloceras (from which 

 I exclude Am. Jamesoni) extends from the Margaritatus- until the Concavum-zoue 

 inclusive. The majority of the species, however, are to be found in the Jurense- 

 and Opalivium-zones — particularly in the top of the former (Dumortieria-beds) and 

 in the base of the latter (Mbo? - ei-beds) ; and these genera, like Grammoceras, help 

 to bind these two zones intimately together, and to show that the Lias- Oolite divi- 

 sion of the German palaeontologists is, after all, extremely arbitrary. 



It is in the Jurense- and Opalinum-zoiies that the genus Dumortieria undergoes 

 that peculiar metamorphosis which causes it to converge towards Grammoceras ,• 

 and the species in these two zones are all very intimately connected together. 

 The isolated species Dum. grammoceroides, which occurs in the Goucavum-zorxe, 

 is just commencing the metamorphosis. It is merely a slight and normal muta- 

 tion of Dum. Levesquei; but in general aspect it has a peculiar resemblance to 

 certain non-spinous species of Sonninia which occur in the same bed. 



1 This is excluding the isolated form Dum. arata ; but see foot-note, p. 233. 



2 The reduction of the suture-line can be traced through the Plates XXXVII and XXXIX— 

 XLIV, and it will be seen to correspond with a broadening of the whorl-area. 



