BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 32, pp. 349-352 SEPTEMBER 1, 1921 



PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY 



CRITERIA FOR SPECIES, PHYLOGENIES, AND FAUNAS OF 



TRILOBITES ^ 



BY PERCY E. RAYMOND ^ 



{Bead before tlie Paleontological Society December 20, 1920) 



CONTENTS 



Page 



Introduction , .349 



Characteristics of tlie body 349 



Cliaracteristics of the cephalou .350 



Characteristics of tlie ventral side and pygidium 351 



Summary 352 



Introduction 



The describe!' of trilobites lias an advantage over the student of 

 Brachiopoda, Mollusea, and various other invertebrates in that the ani- 

 mals with which he deals have more parts, and hence present more com- 

 binations of characteristics which may be used in the discrimination of 

 species. On the other hand, he suffers from the disadvantage that his 

 specimens are ajjt to be dismembered, and very many species are known 

 from fragments only. Due, perhaps, to this latter contingency, trilo- 

 bitists have, as a rule, lieen rather conservative, and it is only within the 

 last few years that an era of nice distinctions has been inaugurated. 

 Compared with the number of individual specimens, the number of spe- 

 cies and genera of trilobites still remains few, but bids fair to be increased 

 rapidly in the next decade or two. 



Characteristics of the Body 



The body of the trilobite is doubly tripartite, but the longitudinal 

 division which is characteristic of the subclass is seldom of specific value. 



1 Manuscript received by the Secretary of the Society March .S, 1921. 

 This paper is one of a series composing a "Symposium on criteria and methods em- 

 ployed in paleontological research." 

 - Read l>y R. S. Bassler. 



(.349) 



