350 p. E. RAYMOND CRITERIA FOR SPECIES OF TRILOBITES 



since the proportion of the width of the axial to the lateral lobes is usually 

 constant in the various genera. The division transversely into cephalon, 

 thorax, and pygidium is subject to much more variation, and the relative 

 lengths of the three portions are very frequently used in separating 

 species. 



The total number of segments in the trunk being highly variable, those 

 trilobites which have many in the thorax usually having few in the 

 pygidium and vice versa, it naturally follows that some uncertainty exists 

 as to the taxonomic value of the number in either. The number in the 

 thorax is constant in some families, as in the Asaphidse and Phacopidia^, 

 but is variable in many others, and even in some species; so that, while 

 it is occasionally cited in specific descriptions, it is only in connection 

 with more important features. 



Chaeactepjstics of the Cephalon 



The cephalon is the part of the trilobite most desired by the taxonomist, 

 for it exhibits the features which are of the greatest value in classifica- 

 tion. It is quite natural that this should be so, for this is ontogenetically 

 the oldest part, and in certain respects it is the fixed and stable portion 

 of the body, to which the trunk is a more or less flexible appendage. Ex- 

 ternal environment will affect more directly the movable thorax and 

 pygidium than the more solid anterior shield. It is, in fact, found that 

 cephala are generally less variable than pygidia, though many specific 

 characteristics are drawn from the head. 



Since the time of Salter and Barrande, the major characteristics used 

 in classification have been those of the cephalon, but it remained for 

 Beecher to show the value of the facial sutures, and so put in logical 

 order the families already in use. As is well known, Beecher used Hyatt's 

 method of inferring relationship from the history revealed by ontogeny, 

 and made it possible, from the inspection of a cephalon alone, to place 

 any particular trilobite in one of the three orders which he proposed. 

 What little work has been done on phylogenies within the group has been 

 based on ontogeny, worked out by Barrande, Matthew, Beecher, Ford, 

 and Walcott, as interpreted by Beecher. 



While the position of the facial suture designates the order and the 

 position in the order of the family, the families themselves are not based 

 on the facial suture, or even on the structure of the head alone, but 

 usually on a number of characteristics drawn from all parts of the body. 

 As now defined, they are, in fact, aggregates of more or less like genera 

 and are in many instances hard to define. This, though bothersome to the 

 taxonomist, is an exceedingly satisfactory state of things, since it shows 



