GEOL.-VOL. r.| SMITH— PLACENTICER AS. 1 93 



using this same method Karpinsky (1889) has been able to 

 correlate the ontogeny of Mcdiicottia and Pronorites with 

 successive ancestral forms, from Anarcestes, Ibergtceras, 

 Paraprolecamtes, 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 (1895) in the ontogeny of 

 the trilobites, all of which are shown to go through a phyl- 

 embryonic protaspis stage, correlative with the primitive 

 crustacean, and similar to the protonauplius of the less 

 specialized living crustaceans. Here, too, it was demon- 

 strated that the larval and adolescent stages of Devonian, 

 Silurian, and even Cambrian trilobites may be correlated 

 with the adults of pre-existing genera, giving the basis of a 

 natural, or biogenetic, classification of this extinct group. 



Many more cases might be added to those cited here, but 

 surely no additional evidence is needed, for all this points 

 in the same direction, whether gathered by believers in or 

 by opponents of the theory of evolution. To this latter 

 class belongs the evidence brought forward by Barrande 

 (1852) in the ontogeny of trilobites, and by Agassiz in the 

 law of recapitulation or acceleration of development. Each 

 of these naturalists used unhesitatingly the method that in 

 the hands of Hyatt and his followers has been so fruitful of 

 results. 



Affinities of Placenticeras and Hoplites. 



The genus Placenticeras was established by F. B. Meek 

 (1876) to include large, discoidal, compressed shells, with 

 narrow umbilicus, narrow, flattened abdomen, and rows of 



