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.]
