144 STUDIES IN EVOLUTION 



There has been much discussion as to the synonymy and 

 value of most of the names proposed as genera or sub-genera 

 in this group. Paradoxides, Remopleurides, and Zacanthoides 

 are about the only ones that have escaped severe criticism in 

 recent years. Taking the type of each of the others, it is 

 found that Elliptocephala (1844) was based on the species 

 E. asaphoides Emmons, Olenellus (1862) on 0. Thompsoni 

 Hall, Mesonacis (1885) on M. vermontana Hall sp., Holmia 

 (1890) on R. Kjerulfi Linnarsson sp., Schmidtia (1890) on 

 S. Mickwitzi Schmidt sp., and Olenelloides (1894) on 0. arma- 

 tus Peach. Some of these names are generally recognized 

 as sub- genera of Olenellus (Mesonacis, Holmia, Olenelloides), 

 while others are considered as synonyms (Elliptocephala, 

 Schmidtia). The early genera were described from very in- 

 complete material, and therefore lacked sufficient diagnostic 

 characters to define them clearly. At the present time 

 nearly or quite entire specimens representing the type species 

 are known, and it is possible to compare all the essential 

 features with some degree of accuracy. The main characters 

 offering the greatest variation are (1) the number of thoracic 

 segments and (2) their specialization into groups, (3) the 

 relative development of the third free segment, (4) the num- 

 ber and position of the spine -bearing segments, (5) the form 

 of the pygidium, (6) the presence or absence of interocular 

 spines, and (7) the form of the cephalon. A simple varia- 

 tion in any one of these would not necessarily imply more 

 than a specific difference, but the genera here mentioned 

 exhibit marked changes in all or nearly all of these charac- 

 ters, and in any family should receive recognition. Olenellus, 

 Mesonacis, and Elliptocephala are more closely related than 

 the other forms, and probably have only a sub-generic value 

 under Elliptocephala. In the first form with fourteen thoracic 

 segments, the third is greatly enlarged and the fifteenth is 

 the spiniform telson-like pygidium. In Mesonacis with 

 twenty-six thoracic segments, the third is somewhat en- 

 larged, and behind the narrow spine-bearing fifteenth seg- 

 ment there are eleven others without spines, followed by the 



