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 integumental 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. Hypopaeia. 

 Family 1. Agnostidae. Family 3. Trinucleidse. 



Family 2. Harpedidae. 



Order B. Opisthopaeia. 



Family 4. Conocoryphidae. ^ Family 8. Bronteidae. 



Family 5. Olenidae. Family 9. Lichadidae. 



Family 6. Asaphidae. Family 10. Acidaspidae. 



Family 7. Proetidae. 



Order C. Peopaeia. 

 Family 11. Encrinuridae. Family 13. Cheiruridse. 



Family 12. Calymmenidae. Family 14. Phacopidas. 



The order Opisthoparia, with nearly one hundred and fifty 

 genera, has a much greater geological distribution than either 



