172 INFERIOR OOLITE AMMONITES. 



sharp ventral area and no defined inner margin — of Gramm. striatulum ; but it has 

 the ornamentation — coarse distant ribs — of Gramm. toarcense. Another form, 

 regarded as belonging more to Gramm. striatulum, shows coarse ribs on its outer 

 whorl, supposed to be a case of reversion. 



Considering all these facts, it is obvious that there is no hard and fast line of 

 distinction between Gramm. toarcense and Gramm. striatulum ; and looking at their 

 relationship we have no right to expect one. The separate names are retained for 

 the extreme forms as a matter of convenience ; and because the uncertain forms 

 are after all not very numerous. 



A form of Gramm. toarcense with joined ribs is figured by Haug (loc. cit.) as 

 Gramm. striatulum var. comptum. I have seen one or two specimens with some- 

 thing of this style of ornamentation, but have not been able to obtain them for 

 figuring. A small specimen, remarkable for its very open umbilicus, broad ventral 

 area, and occasional joined ribs I have had depicted, PI. XXXIV, fig. 12, but this does 

 not show the character like Haug's figure. These joined ribs, which we trace in 

 the young of Gramm. doerntense and Gramm. aalense, must have become a definitely 

 fixed character attached to a particular adult form, and specimens of this kind would 

 require a separate name. However, all that my specimens seem to indicate is the 

 prior stage — the time when the character was by no means constant, and when it 

 appeared casually in any varieties of Gramm. toarcense. At present I cannot 

 separate any of my specimens under a definite name on account of this character ; 

 but possibly the specimens which Haug has obtained are actual intermediate links 

 between Gramm. toarcense and Gramm. doerntense. 



For Hildoceras horeale^ this species might sometimes be mistaken, especially 

 when the specimens are small ; and since this mistake might involve incorrect 

 geological inferences — H. horeale occurs in the Commune-zone or possibly deeper 

 — it is necessary that it should be avoided. A greater bend of the ribs upon the 

 lateral area, more persistent ventral furrows, a more pronounced — in adult, concave 

 inner margin — accompany some differences in the suture-line and render Hildo- 

 ceras horeale distinct from Gramm. toarcense. 



With. Dumortieria Levesquei{di'Orh\gnj)it is possible that Gramm. toarcense might 

 be confounded ; but the same difference which separates the latter from Hild. horeale 

 separates Diiin. Lcvesquei from Gramm. toarcense ; and this is straighter ribbing on 

 the lateral area. This feature, accompanying a great difference in the suture line, 

 distinguishes Dum. Levesquei from Gramm. toarcense. 



The name of this form was originally written by d'Orbigny as Thouarsensis ; 

 and herein he has been followed by all authors except E. Deslongschamps, who 

 wrote Ammonites toarcensis and Lima toarcensis. Since the name is taken from 

 Thouars, a town in Deux Sevres, of which the correct Latin name is Toarcium, it 



1 Harpoceiaa Lcvisoni, Wright (iiuu Simpson), ' Mouog. Lias Amm.,' pi. Ixi, figs. 5, U only. 



