THE CEPHALOPODA 181 



faunas, the gaps in the phylogenetic sequence are im- 

 pressive. Neumayr, forty years ago, first called special 

 attention to this in a famous essay. He pointed out how 

 at certain geological horizons there appeared abundant 

 new ammonites which could not be the descendants of 

 any in the earlier zones : these he termed crypto genetic 

 types. In other cases, similar forms reappeared at wide 

 intervals, while absent from intervening strata : these he 

 termed sporadic types. Two particular cases of the 

 latter he was able to explain very satisfactorily : at 

 various horizons in the Jurassic of South Germany (and 

 at some but not all of the same horizons in England) 

 there appear species of the genera Phylloceras and Lyto- 

 cevas (or, as we should now express it, of the families 

 Phylloceratidae and Lytoceratidae). But similar forms 

 occur throughout the Jurassic of the Mediterranean 

 region ; hence Neumayr's explanation of their sporadic 

 occurrence farther north was that opportunities for migra- 

 tion occurred occasionally owing to geographical or 

 climatic changes, and he expressed the belief that other 

 sporadic and cryptogenetic types would prove to have 

 migrated from unknown regions. Since Neumayr wrote 

 it has been shown that many of his supposed " sporadic " 

 types are not related to one another but are homoeo- 

 morphs, as for instance various oxycones which at that 

 time were placed on account of their form in the genus 

 A maltheus. 



Cryptogenetic types are indications of the imperfection 

 of the palaeontological record, which, as Mr. Buckman 

 points out, may be due to several causes : 



