112 STUDIES IN EVOLUTION 



ences, and the latter adopts a division into two lines, based 

 on the number of body rings and the size of the pygidium. 

 These include and are themselves included in four groups, 

 founded on the presence and form of the facial suture and 

 the structure of the eyes. 



Haeckel*has recently given the trilobites their full value 

 in a classification of the articulates. Although he has not 

 advanced a detailed classification, still it is desirable to 

 review the ordinal groups which he proposes. He considers 

 the Trilobita as a legion under the first class, Aspidonia, of 

 the Crustacea, which is characterized as being without a 

 nauplius larval form and as having a pair of pre-oral antennae. 

 In this class is also included the legion Merostomata, the 

 Trilobita being especially distinguished by the number and 

 character of the legs. The writer 5 believes that it is now 

 satisfactorily demonstrated that the protaspis, or early 

 larval form of the trilobite, is a protonauplius, and homol- 

 ogous with the nauplius of higher Crustacea. Therefore 

 the Trilobita cannot remain in the Aspidonia, as here 

 defined. 



Haeckel further divides the Trilobita into two orders, the 

 first, the Archiaspides (or Protrilobita), and the second, the 

 Eutrilobita (or Pygidiata). The Archiaspides are represented 

 by the families Olenida and Triarthrida, and are distinguished 

 by the absence of a real pygidium, and by the complete 

 homonomy of the numerous body segments and their phyllo- 

 podiform appendages. The families are themselves distin- 

 guished by the semi-circular or crescent-shaped cephalon and 

 by the presence or absence of genal spines. The Eutrilobita 

 are represented by the families Asaphida and Calymmenida, 

 and are marked by the heteronomy of the body segments as 

 expressed in the functional pygidium. 



Salter, Burmeister, and Emmrich have, as previously 

 noticed, attempted to use the comparative size and develop- 



* Systematische Phylogeiiie der wirbellosen Thiere (Invertebrata). Zweiter 

 Theil, 1896. 



