, 
82 GENESIS OF THE ARIETID™. 
marked characteristic of the umbilicus. The sutures are also peculiar in the 
simplicity of their marginal outlines and proportions, and these peculiarities 
remain constant. 
In the adult and young of some varieties of the radical species of the coroni- 
ceran series, Qor. kridion, a form appeared having strongly divergent sides, lyre- 
shaped tuberculated pila, sutures with deep abdominal lobes and prominent 
inferior lateral saddles, while in the young of other varieties there was a nearer 
approximation to the young of Arnioceras. In all the succeeding species of the 
series except Cor. Sauzeanum, a direct descendant of kridion, this divergent-sided, 
broad-abdomened whorl was found at an early nealogic stage, having the same 
lyre-shaped pile, deep channels, and arietian sutures, 
On looking back, we see that Arn. muiserabile, semicostatum, kridioides, and Cor. 
kridion, may be considered as a series in which kridion was a terminal species with 
an accelerated development in some varieties, and that from this last highly 
specialized form arose, as we have stated above, the species of the highly pro- 
gressive coroniceran series, the typical acmic series of the Arietidse. The arie- 
tian differentials, the long abdominal lobe and prominent inferior lateral saddles, 
and the combination of these with the quadragonal whorl, highly developed 
keel, channels, and geniculated and tuberculated pil, were barely indicated in 
the caloceran series, and appeared in perfection only in the higher species of 
Vermiceras. Although they were generated with great rapidity in the arnioce- 
ran series, yet they were present in full perfection and were comparatively 
constant only in the species of the coroniceran series, which, as we have said, 
were directly derived from Cor. kridion, a species in whose adults these characters 
first appeared in their final arietian shape and proportions. 
The remaining series, which can be properly called the geratologous genera 
of the Levis Stock, form a distinct group composed of a central series and three 
lateral series, offshoots from the common radicals, Agas. levigatum and striaries. 
The necessary mode of arrangement places Asteroceras on the left, Agassiceras 
in the centre, and Oxynoticeras' on the right. The structural characters: also 
agree with such an arrangement. No progressive linear series can be formed 
out of the radical species of these series, as in the genera mentioned above the 
arrangement is necessarily radiatory like the spokes of a fan. The differentials 
of the adult of the radical species Agas. devigatum were quite constant in the 
species ; we refer to the discoidal smooth whorls and fold-like pile, the simple 
but arietian sutures with their deep abdominal lobe and prominent inferior lateral 
saddles. The shell also had fewer whorls and shorter living chambers than the 
adult of Psiloceras planorbe. In Agas. striaries there is close similarity to daviga- 
tum, but very distinct strie and a larger size. In Agas. Scipionianum, the prominent 
keel, channelless abdomen, pila, and tubercles were abruptly introduced, and 
were the principal differential characteristics which distinguished the series from 
all others in the Arietidas. This abrupt introduction indicates the former exist- 
ence of intermediate forms which remain to be discovered. 
It may be that a true hollow keel may have appeared in Scipionianum, as is 
1 Summ. PI. xiii. and xiv. 
