78 GENESIS OF THE ARIETID^E. 



ragonal form, or rather an immature representation of it, occasionally occurred 

 in some adult individuals of Agas. Icmgatvm, in which the sides were flatter 

 than usual. In Agas. striaries the quadragonal form and the siphonal line or 

 keel were more decidedly expressed, as well as the tendency to elevate the abdo- 

 men. In Agas. Scipionianum after the earlier nealogic stages were passed which 

 closely resembled the full grown of striaries, with the exception of the thicker 

 pil-e and somewhat deeper umbilicus, the adult showed a quadragonal whorl 

 with a keeled abdomen and tuberculated pilaa. The old age had a smooth trigo- 

 nal whorl. In Agas. Scipionis, 1 which is a naturally distinct form, the extreme 

 varieties had more involute whorls, smooth pilas, and became trigonal and 

 smooth at an early stage. 



Thus, at all stages of growth and decline, the correspondence or parallelism 

 between the individual and the morphogeny of the series is complete. 



In the second subseries of Oxynoticeras we have found that there was one 

 species, Oxyn. Lotharingum, in which the whorl during the last senile stage became 

 completely rounded on the abdomen. The sides became gibbous and narrower, 

 thus showing a slight tendency to revert to the primitive form of the less dis- 

 coidal Agas. striaries and Psil. planorbc. These similarities were also greatly in- 

 creased by the appearance of senile folds similar to the primitive pilations of this 

 species and Psiloceras. The adults of the species of this subseries were also gera- 

 tologous, in so far as the forms were not only much compressed and trigonal, but 

 also smooth. The degeneration and the total loss of the hollow keel also oc- 

 curred in this oldest stage. We should not be at all surprised if species should 

 be found, and identified as belonging to this series, in which the hollow keel 

 was either not present at any stage, or was only slightly indicated during the 

 nealogic stages. These adults would then correspond to the geratologons stage 

 of Oxyn. Lotharingum, in the same way that Ast. Colhnoti corresponded to the 

 old of the normal forms of Ast. obtusion and steilare. 



If a tendency to the inheritance of retrogressive characters be granted, 

 and certainly their occurrence at earlier stages in successive species makes 

 this view seem highly probable, then the same law of replacement which pro- 

 duced progression would now act upon successive organisms so as to produce 

 retrogression. The observed phenomena indicate the direct replacement of 

 the characters of progressive ancestors by degenerate characters, which were 

 first observable in the old age of these ancestors themselves. If there had 

 been in most cases simply a mass of degenerate forms, without any defina- 

 ble evidences of successive gradations, as in the famous instance of the Magnon 

 examples of distorted Planorbidse, it would be possible to say at once that 

 the parallelisms of the geratologons period, with retrogressive characters in 

 what we have called geratologous species of the same series, were purely homo- 

 plastic correspondences. On the contrary, the gradations are perfectly well 

 marked, as we have described them above and in the Introduction to this mono- 

 graph, and the replacement of progressive characters by the geratologous takes 

 place in strict accordance with the law of acceleration in heredity. 



1 Summ PI. xiii. fig. 8. 



