114 STUDIES IN EVOLUTION 



any species are completely in discordance with these classifi- 

 cations, and clearly demand other interpretations. 



Rank of the Trilobites. 



As to the rank of the trilobites in a classification of the 

 Crustacea, there is also much diversity of opinion. They 

 have long been regarded as an order, but any attempt to 

 include them in this way under higher groups, such as the 

 Entomostraca, Malacostraca, or Palsoocarida, results in such 

 broad generalities and looseness of definition as to render 

 these divisions of little value. Even the Entomostraca, as 

 restricted to the orders Phyllopoda, Ostracoda, Copepoda, 

 and Cirripedia, seem heterogeneous and probably polyphy- 

 letic. Milne-Edwards, 27 Gegenbaur, 19 Walcott, 33 and others 

 have considered the trilobites as belonging to a class of 

 arthropods intermediate between the Crustacea arid arach- 

 nids. Some recent authors, as Lang, 24 have attempted to 

 overcome the difficulty by attaching them as an appendage 

 (" Anhang ") to the Crustacea. Kingsley, 2S on the other hand, 

 has placed them as a sub-class of the Crustacea, leaving all 

 the other Crustacea to come under a second sub-class, the 

 Eucrustacea. The present state of knowledge of their struc- 

 ture and development is in favor of giving the trilobites the 

 rank of a sub-class, but for purposes of comparison and corre- 

 lation the fullest results can be brought out by recognizing 

 the old and well-known sub-classes, the Entomostraca and 

 Malacostraca. 



The following tabular view of the leading points of the 

 comparative morphology of the three sub-classes is intro- 

 duced to show, first, the claims of the Trilobita as an equiv- 

 alent group, and, second, the progressive differentiation of 

 characters. In nearly every particular the trilobite is very 

 primitive, and closely agrees with the theoretical crusta- 

 cean ancestor. Its affinities are with both the other sub- 

 classes, especially their lower orders, but its position is not 

 intermediate. 



