424 JAMES P ERR IN SMITH 



a nuculoid stage, homologous with the primitive radicle of the 

 group. Every Pecten goes through stages successively correla- 

 tive with a nuculoid, Rhomb opteria, Pterinopecten and Aviculopecten 

 before it reaches maturity, each stage appearing in the order of 

 the ancestral genus. Even the greatly modified oyster shows 

 its kinship with this group by its nuculoid and Rhombopteria 

 stages. 



The researches of Branco have made it clear that each group 

 of cephalopods has its typical phylembryo, in a general way 

 correlative with the radicle of the group, and that the later 

 stages may be compared very accurately with ancestral families 

 and genera. The way for this was opened by Hyatt's memoirs 

 on the ontogeny of the ammonites, in which it was shown that 

 in each perfect adult ammonite shell the complete individual 

 ontogeny is recorded. By using this same method Karpinsky 

 has been able to correlate the ontogeny of Medlicottia and Pro- 

 norites with successive ancestral forms, from Anarcestes, Ibergi- 

 ceras, Paraprolecanites, up to the adult stage. 



By the ontogenic method Buckman has been able to get at 

 a sound basis of classification of the Jurassic ammonites, and to 

 correlate the growth stages of many of these with their race 

 history. Although his conclusions as to the systematic position 

 of many of these genera do not agree with the ideas commonly 

 accepted concerning them, it must not be forgotten that these 

 conclusions are based, not merely on ontogenic study alone, but 

 also on the gradual transitions of a series of adults. This is the 

 strongest confirmation that any phylogenic research could ever 

 have. 



Crustacea. — Among the most convincing morphogenic 

 researches are Beecher's studies in the ontogeny of the trilo- 

 bites, all of which are shown to go through a phylembryonic 

 protaspis stage, correlative with the primitive crustacean, and 

 similar to the protonauplius of the less specialized living crus- 

 taceans. Here, too, it was demonstrated that the larval and 

 adolescent stages of Devonian, Silurian, and even Cambrian 

 trilobites may be correlated with the adults of preexisting 



