130 INFERIOR OOLITE AMMONITES. 



group of Am. Sowerbyi (Sonninia, Bayle, Hammatoceras pars, Zittel) are, in my 

 opinion, descended from some form allied to Amaltheus, and must, therefore, 

 naturally be kept separate ; while as to Oppelia, although its descent is not yet 

 clear to me, it is obviously not derived from Arietites. In the genus Harpoceras 

 have also been included other species besides those which would come under any 

 of the above heads, namely, the species which form the group of Am. Levesquei 

 (Dumortieria), and which, owing to great convergence in shape towards Grammo- 

 ceras, have been hitherto classed in the genus Harpoceras ; l but Dr. Haug 2 has 

 traced their descent through Dumortieria Jamesoni, and Polymorphites, to Agassi- 

 ceras of the Lower Lias, and has placed them in his family Polyrnorpliidce. 



Now, in order to show at a glance the descent of the Hildoceratidce, which has 

 such an important bearing upon this subject of classification, I append the 

 following Tables II and III. The first is intended to give some idea of the 

 descent of the various species in the first four genera, and so to demonstrate how 

 the material for Table III has been worked up. 



In Table II we may notice in passing that the genus Ludivigia is composed of 

 two branches — the dwarf Ludivigice (L. costosa, comu, rudis), and the larger 

 Ludwigice (L. MurcMsonce, Lucyi, 8fc). The former seems to end with the 

 Ocmcavwm-beds ; the latter, although it disappears temporarily from view above the 

 Concavum-heds t without doubt reappears later on — the Hectici being the direct 

 descendants of the larger Ludivigiai. Pseudolioceras seems to come to an end 

 in the Opalinum-zone, but had shortly before given birth to the forms which 

 compose the genus Hyperlioceras. A long distance separates the first and second 

 appearance of the genus Lioceras ; and I imagine that the parent-forms of Lioceras 

 hradfordense and Live, f alias? were in existence during that period. Lioc. ambiguum 

 I have not put down. It is a species of which the descent is obscure ; and it may be 

 a convergence from another series. 



These Tables are probably self-explanatory. They are an attempt to show what 

 conjectures may reasonably be formed concerning the relationship of the various 

 species and genera of the Hildoceratidce in accordance with the geological positions 

 in which we find them. They require, of course, to be read together, and also in 

 conjunction with a Table which I shall give presently for Branch C — Grammoceras ,- 4 

 and if that be done the position of the various species in the same genus with 

 regard to other genera will be seen at a glance. 



1 That is before the genus was restricted. 



2 " Ueber die Polymorphidse," ' JSeues Jahrbuch fur Mineralogie, &c.,' Bd. ii, 18S7. 



3 That is to say, that these species bave not come through Lioc. opalinum, but from certain forms 

 which had diverged from the parent-stem before Lioc. opalinum appeared. 



4 I have thought that the genera Hildoceras, Lillia, &c, are not sufficiently within the limits of 

 this Monograph to demand a Table. 



