490 a. .t. jukes-browne's supplementary 



In size, number of chambers, and position of siphuncle they 

 resemble iV. Bouchardianus ; but the septa of the latter are described 

 as straight. 



Ammonites rhamnonotus, Seeley. 



A. rhamnonotus, Seeley, Ann. & Mag. Nat. Hist. ser. 3, xvi. 

 p. 233, pi. xi. fig. 7. 



A. navicularis, var. nothus, Seeley, torn. cit. p. 232. 



A. gardonicus, Hebert & Munier-Chalmas (Bassin d'Uchaux), Ann. 

 des Sc. Geol. tome vi. pi. 4. figs. 1, 2. 



Tn 1875 (Quart. Journ. Geol. Soc. vol. xxxi. p. 28G) I pointed out 

 that the fossils indicated by the second of the above names had a closer 

 relationship with A. Mantelli than Avith A. navicularis, but had also 

 some characters in common with A. dispar, D'Orb., as figured and 

 described by MM. Pictet and Campiche. 



In the same year Prof. Hebert published his description of the 

 Bassin d'Uchaux, in the appendix to which he described and figured 

 the young and old forms of an Ammonite from the Gault of Yal- 

 bonne under the name of A. gardonicus. The resemblance 

 between these and some of the Cambridge specimens is very great, 

 the young form being very like the A. rhamnonotus of Mr. Seeley, 

 while the older individual approaches nearly to that called by him 

 A. navicularis, var. nothus. A comparison of the descriptions given 

 by Prof. Hebert and Prof. Seeley respectively will show how closely 

 A. rhamnonotus and A. gardonicus are allied. After describing 

 the young form, M. Hebert observes, "in older individuals the 

 ribs pass over the back, and only present three slight elevations, 

 which sometimes finish by disappearing altogether." 



Mr. Seeley says : — " On the back the ribs are rather less distinct, 

 and each bears in its centre a small sharp tubercle. In a younger 

 state there are also tubercles at the extreme edge of the back, 

 which seem to disappear with a diameter of twelve lines." 



Another common characteristic is that only some of the ribs 

 spring from the umbilicus, the intermediate ones arising from near 

 the middle of the whorl ; this, however is also the case with the 

 ribs ornamenting the so-called A. navicidaris, var. nothus ; and there 

 are specimens now in the Woodwardian Museum which plainly con- 

 nect this with the former. In one of these the earlier ribs are fre~ 

 quent and rounded, while the later ribs are wider apart and become 

 rudely nodulated ; in others the tubercles appear to be lost with 

 age, as in Prof. Hebert 's specimen ; another form resembles the 

 fragment named A. Wiestii by Sharpe. In view, therefore, of 

 the great variation exhibited in this series of specimens, I propose 

 that they be all considered as varieties of one species, for which 

 Mr. Seeley 's name A. r*hamnonotus has the prioritv. 



[Since the above was written I find that Mr. Seeley has obtained 

 casts of A. gardonicus and placed them in the Museum, identi- 

 fying them at the same time with his A. rhamnonotus; this identity 

 may therefore be regarded as established.] 



