352 p. E. RAYMOND CRITERIA FOR SPECIES OF TRILOBITES 



taxonomic rank. The proportion of length to breadth is almost always 

 constant in small groups and has been used to advantage in making spe- 

 cies in various genera, particularly of smooth trilobites, which lack other 

 striking features. Correlated with this is the number of segments, which 

 is seldom fixed and may or may not be of specific value. In some genera, 

 where there is wide variation in this particular, a difference of five or six 

 will pass unnoticed, while in others new species have been erected because 

 of one more or less than the usual number. The rings on the axial lobe 

 represent the true number of segments in this shield, and there are 

 usually less ribs on the pleural than rings on the axial lobe. The number 

 of ribs is frequently of specific importance, as is also their length, promi- 

 nence, furrows, and ornamentation. The width of the border should also 

 be noted, and while its presence or absence may be of generic importance, 

 its width, outline, and profile often serve to limit a species. When spines 

 or pustules are present, either on the margin or on any portion of the 

 upper surface, their position, size, curvature, arrangement, and number 

 almost always present combinations of specific value. 



Summary 



It is obvious from the above outline that the trilobites do not lend 

 themselves readily to an artificial classification, and that there are no 

 uniform "generic" or "specific" characteristics. It can not be said that 

 an ideal and complete classification has yet been reached, and only when 

 a fuller understanding of the structure and ancestry of the whole group 

 is attained will there come full realization of the relative importance of 

 the various parts of the anatomy. We are, in our descriptions, still 

 struggling somewliat blindly toward the goal. 



