S2 GENESIS OE TPIE AEIETID.E. 



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, Cor. kridion, a form appeared having strongly divergent sides, lyre- 

 shaped tuberculated pite, 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 pilaa, deep channels, and arietian sutures. 



On looking back, we see that Am. miserdbUe, semicostafum, 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 acinic series of the Arietidao. The arie- 

 tian differentials, the long abdominal lobe arid prominent inferior lateral saddles, 

 and the combination of these with the quadragonal whorl, highly developed 

 keel, channels, and geniculated and tuberculated pilse, 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. Iwvigatnm and striaries. 

 The necessary mode of arrangement places Asteroceras on the left, Agassiceras 

 in the centre, and (Xvynoticeras l 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. Icevigahun were quite constant in the 

 species ; we refer to the discoidal smooth whorls and fold-like pilae, 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 phnorbe. In Agas. striaries there is close similarity to Iceviga- 

 tum, hut very distinct stria? and a larger size. In Agas. Scipionianum, the prominent 

 keel, channelless abdomen, pilae, and tubercles were abruptly introduced, and 

 were the principal differential characteristics which distinguished the series from 

 all others in the Arietidse. 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 Svipionianum, as is 

 1 Suram PI. xiii. and xiv. 



