7926 SYSTEMATIC 23 



forms showing, as adults, the epacmic or anagenetic characters. There- 

 fore the earlier forms cannot be regarded as ancestral to the later forms, 

 which is the same as was found in the Oppelaceae ; so geological order 

 is, in the main, the inverse of biological order, and the successive waves 

 are only related, not directly to one another, but indirectly as constantly 

 originating from some unknown but persistent common ancestor. Such 

 ancestor may reasonably be conjectured to have arrived at the Telo- 

 ceras-Stepheoceras stage of cadicone coronate, and to have abided in 

 this form in its unknown locality. Perhaps occasionally it branched out 

 into the Parkinsonian broken-venter stage. But the Parkinsonian 

 biological stage has only unituberculation — on the side ; while the 

 Kosmoceratids have bituberculate side and unituberculate venters — 

 trituberculation. So there are two stages between Parkinsonians and 

 Kosmoceratids, bituberculation and trituberculation. The Kosmocera- 

 tids pass through trituberculation before declining : there is no evidence 

 that the early Kosmoceratacese — the Gowericeratidae — all passed through 

 the trituberculate, or even the bituberculate stage before declining : 

 it is possible that they were aiming in such directions, but failed in 

 attainment. In such case there is only a kind of Parkin.sonian biological 

 history before the advent of the cryptogenetic Cerericeras : in the case 

 of attainment there would be a long Kosmoceratid kind of history before 

 it, involving a far greater series of missing forms. 



An interesting problem is presented by the disappearance of 

 Kosmoceratacese during the Reineckeian Age ; but it is paralleled to a 

 certain extent by the sporadic appearances and disappearances of 

 Lytocerates and Phyllocerates in the seas of northern Europe. Partly, 

 but not completely, such gaps in their continuity are filled by records 

 from more southerly areas. It is possible to understand that denizens 

 of cold-water areas would invade warm-water areas and suddenly 

 flourish ; because this is in accordance with usual biological phenomena. 

 But it is not easy to understand the inhabitants of warm seas invading 

 cold seas, except on the postulate that the cold seas became temporarily 

 warm owing to climatic or physical changes. In any case, however, 

 there seem to be long gaps in continuity of record, for which the only 

 explanation would appear to be that comparatively primitive forms 

 lay, more or less dormant, in unknown or untapped areas, ready to 

 take advantage of favouring recurrent conditions to spring into flourishing 

 development — such development necessarily nearly parallel to what 

 had gone before. But the degree of such parallelism depended on the 

 degree of development of the dormant forms. If they were more or 

 less developed along a given line they would produce somewhat close 

 parallelism ; but if they were decidedly primitive they would not be 

 set on a given course, but would be more capable of launching out 

 into quite new directions. 



Some such theory is necessary to account for the reappearances 

 after temporary absence of forms which, from their essential similarity, 

 appear to be actually related, as distinguished from those forms which 

 are superficially alike, but are obviously not related. 



A few examples of the re-appearance of presumably related forms, 

 after a more or less prolonged absence, may be given. For the time- 

 scale see T.A. V, pp. 71-78. 



HapLopleuroceras in Sonninian 

 Paltoplenroceras in Amaltheian 



