132 STUDIES IN EVOLUTION 



of the others, and was by far the dominant group during the 

 Cambrian and Ordovician, being represented by about eighty- 

 five genera in the former age and forty-five in the latter. 

 Nineteen genera of this order are found in the Silurian and 

 ten in the Devonian, most of them having continued on from 

 older ages. Four genera represent the order in the Car- 

 boniferous and one in the Permian, thus marking the ex- 

 tinction of the sub-class as well as the last genera of the 

 Opisthoparia. 



The comparative abundance and duration of the three 

 orders are expressed in the table on page 133, from which 

 it appears that the Hypoparia probably culminated in pre- 

 Cambrian times, the Opisthoparia during the Cambrian, and 

 the Proparia during the Ordovician. 



In the following classification the families adopted by 

 Salter ^^ and Barrande ^ are in the main adhered to, and the 

 number corresponds ver}^ closely with that in Zittel's " Hand- 

 buch der Palseontologie " ^^ and also in the " Grundzuge" '''^ of 

 the same author. The order of arrangement, however, is 

 very different. A great number of family divisions have 

 been proposed, and undoubtedly many others will yet be 

 made, but it is not within the province of this paper to deter- 

 mine the precise value and limitations of the families. This 

 would require discussions of priority and synonym}', and other- 

 wise detract from the direct purpose of the writer; namely, 

 to establish a basis for a natural classification, and in this 

 way to apply what is currently known and accepted regard- 

 ing the trilobites. Nevertheless, some notice must be taken 

 of several families and genera which for various reasons do 

 not appear here. The family Aglaspidse, including the genus 

 Aglaspis Hall, proves to belong to the Merostomata and is 

 therefore omitted. The family Bohemillidse has been shown 

 by the writer ^ to have no foundation, because the type of 

 the genus Bohemilla Barrande was based upon a mutilated 

 specimen of ^glina. 



Several genera still commonly adopted are not here recog- 

 nized in the lists under the families, since from the minute 



