NATURAL CLASSIFICATION OF THE TRILOBITES 133 



size of the individuals described and their immature char- 

 acters they must be considered as the young of larger forms. 

 Such are Gonophrys Callaway, Cyphomscus Salter, Holo- 

 ■metopus Angelin, Isocolus Angelin, and Shumardia Billings. 

 Triopus Barrande has been shown to be a chiton. 



. 75 



Permian. 



Carboniferous. 



Devonian. 



Silurian. 



Ordovician. 



Cambrian. 



Pre-Cambrian. 



FiGDEB 75. — Table of Geological Distribution of Trilobita. 



Much could be said against some of the recognized genera, 

 but, as with the families, the writer has preferred in almost 

 every case to adopt, for the present, what has been commonly 

 accepted, and thus to avoid the entanglement of dates and 

 synonyms which would be out of place in any general discus- 

 sions. The type species of every genus is here made the 

 central idea. It is taken as representing the genus more 

 closely than any fortuitous assemblage of diverse species, 

 which the next investigator may show belong to another or 

 to several genera. Our ideas of a genus are naturally based 

 mainly upon the species with which we are most familiar. 

 Until forced to make authoritative comparative statements, 

 it does not occur to one that the type of the genus under 

 consideration may be quite different. An American stu- 

 dent's conception of Homalonotus will probably be formed 



