CLASSIFICATION OF THE TRILOBITA 



In elaborating a natural classification of any group of or- 

 ganisms, a classification which will show the genetic relation- 

 ships of the various forms, the best results have always been 

 attained through the application of the law of morphogenesis 

 which supposes that each individual, during its development 

 from embryo to adult, passes through a series of stages which 

 more or less closely resemble the stages through which the 

 ancestors of this individual have passed in preceding ages. 

 The principles of this law can best be applied in the classifica- 

 tion of a group of organisms not made up of degenerate forms, 

 and whose geologic history is more or less complete. The 

 trilobites have a known life history beginning with the Cam- 

 brian and extending to the close of the Paleozoic, their structure 

 is generalized and quite uniform throughout, no degenerate 

 .forms are known, and they satisfy most of the conditions nec- 

 essary for the satisfactory application of the law of morpho- 

 genesis in building up a natural genetic classification. 



In common with all organisms known only in the fossil 

 form, the trilobites . furnish almost no information in regard to 

 the strictly embryonic stages of growth, although minute 

 spherical or ovoid bodies have been found associated with trilo- 

 bites and have been described as the eggs of these creatures, 

 The smallest and most primitive fossil organisms exhibiting the 

 characters of the trilobite, and which have been traced through 

 successive stages to the adult, are small discoid or ovate bodies 

 named the protaspis, and do not exceed i mm. in length. The 

 protaspis stage has been found to be the normal larval form 

 characteristic of all trilobites, and it is believed that it approxi- 

 mates the theoretical, primitive, ancestral larval form of the 

 Crustacea. The characteristics possessed by the protaspis, 

 as now known in all the principal groups of trilobites, are given 

 by Beecher as follows: "Dorsal shield minute, not more than 

 0.4 to i mm. in length; circular or ovate in form; axis distinct, 

 more or less strongly annulated, limited by longitudinal grooves; 

 head portion predominating; axis of cranidium with five an- 

 nulations; abdominal portion usually less than one-third the 

 length of the shield ; axis with from one to several annulations ; 

 pleural portion smooth or grooved; eyes, when present, ante- 

 rior marginal or submarginal; free cheeks, when visible, nar- 

 row and marginal." 



