CHARACTERISTICS OF THE CEPHALON 351 



that there is a gradation from one group to another, and that material 

 is gradually accumulating that will permit the evolution of the subclass 

 to be followed out. Genera, as they are now coming to be limited, are 

 merely small aggregates of similar species, and various men hold widely 

 different ideas as to the latitude allowable in a generic group. 



Generic characteristics are generally drawn from the features of the 

 whole body, if all parts are known ; otherwise from the head. It is true 

 that a very few genera have been based on pygidia alone, through force 

 of lack of material, but not from choice. It is often impossible to deter- 

 mine a genus from the latter shield alone, and in cases of parallelism the 

 distinguishing characteristics are usually to be found on the cephalon. 

 A few genera have been based in part on the form of the thorax; but, 

 while that may be diagnostic within a genus, it is not often relied on 

 entirely. 



Specific value is usually given to such features of the cephalon as the 

 proportion of length to breadth of both the whole shield and of the gla- 

 bella; sometimes to the position of the eyes, although this is often of 

 higher rank ; to the length and shape of the genal spines ; to the brim, its 

 extent, profile, and breadth; the convexity of the glabella and the condi- 

 tion of its furrows; the position and characteristics of any spines or 

 pustules; and to the pattern of any ornamentation that may be present. 

 All these things have different values in different groups, and, so far as 

 possible, combinations of characteristics are chosen. At the present time, 

 trilobites are usually described in considerable detail, for one is never 

 quite sure which are the features which are going to be of most use. The 

 short descriptions in Latin given by some of the earlier writers are fre- 

 quently of no value at all, as they may apply to a number of species now 

 known. 



ChAEACTERISTICS of THE VENTRAL SiDE AND PyGIDIUM 



The ventral side of the body has never, so far as I know, been used in 

 discriminating species. The extent and form of the doublure has in a 

 few cases afforded generic distinctions; the shape of the epistoma is, in 

 many cases, of family value, and the hypostoma often enables one to rec- 

 ognize a genus or, if not that, a family. The appendages, even if they 

 were commonly well preserved, would probably be of little help in making 

 other than the larger subdivisions, for they seem to have been generalized 

 and very similar in unlike trilobites. 



The pygidium is more used in making specific distinctions than any 

 other part of the body. In most cases the minor details of the conforma- 

 tion of this shield are so variable that the whole combination has low 



