NATURAL CLASSIFICATION OF THE TRILOBITES 131 



Somites of the thorax movable upon one another, varying in 

 number from two to twenty-nine. Abdominal segments vari- 

 able in number, and fused to form a caudal shield. All seg- 

 ments, thoracic and abdominal, carry a pair of jointed biramous 

 limbs. All limbs have their coxal elements forming gnath- 

 obases, which become organs of manducation on the head. 

 Respiration integumeutal and by branchial fringes on the exop- 

 odites. Development proceeding from a protonauplius form, 

 by the progressive addition of segments at successive moults. 



Heretofore it has been impossible to give an adequate 

 diagnosis of the Trilobita, owing to the absence of informa- 

 tion regarding certain important characters, and the obscurity 

 of the information relating to some other features. It is 

 believed that enough is now known to frame a definition of 

 the class, which, in accuracy and completeness, will compare 

 favorably with any based upon living groups. Such a defi- 

 nition brings out the fact that the differences between the 

 trilobites and other large groups are clearly recognizable, and 

 do not consist of a statement of anomalous characters whose 

 real significance is unknown. 



Arrangement of the Families of Trilobites. 



Sub-class TRILOBITA. 



Order A. Hypoparia. 



Family 1. Agnostidae. Family 3. Trinucleidse. 



Family 2. Harpedidae. 



Order B. Opisthoparia. 



Family 4. Conocoryphidse. , Family 8. Bronteidse. 



Family 5. Olenidse. Family 9. Lichadidae. 



Family 6. Asaphidae. Family 10. Acidaspidae. 



Family 7. Proetidae. 



Order C. Proparia. 



Family 11. Encrinuridae. Family 13. Cheiruridae. 



Family 12. Calyramenidi^e. Family 14. Phacopidae. 



The order Opisthoparia, with nearly one hundred and fifty 

 genera, has a much greater geological distribution than either 



