122 STUDIES IN EVOLUTION 



and gastropods; the prodissoconch, in the pelecypods; the 

 protegulum, in the brachiopods, and the protechinus, in the 

 echinoids. In the trilobites the protaspis, as already stated, 

 has the value of the phylembryo, and in its geological history 

 and the metamorphoses it undergoes to produce the perfect 

 trilobite accurate information can be gained as to what the 

 primitive characters are, and the relative values of other 

 features acquired during the long existence of the class. 



The simple characters possessed by the protaspis are the 

 following, as drawn from the study of this stage in all the 

 principal groups of trilobites : Dorsal shield minute, not more 

 than .4 to 1 mm. in length; circular or ovate in form; axis 

 distinct, more or less strongly annulated, limited by longi- 

 tudinal grooves; head portion predominating; axis of cranid- 

 ium with five annulations; 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, anterior, marginal, or sub-marginal ; free- 

 cheeks, when visible, narrow and marginal. Examples, 

 Plate II, figures 1, 5. 



During this stage several moults took place before the 

 complete separation of the pygidium or the introduction of 

 thoracic segments. These brought about various changes ; as 

 the stronger annulation of the axis, the appearance of the 

 free-cheeks on the dorsal side, and the growth of the pygid- 

 ium by the introduction of new appendages and segments, 

 as indicated by the additional grooves on the axis and limb. 

 A full representation of the variety and succession of these 

 early protaspis stages is presented in the writer's paper on 

 the "Larval Stages of Trilobites." 6 Some of the conclusions 

 and discussions in that paper are made use of here. 



In the earliest or Cambrian genera the protaspis stage is 

 by far the simplest expression of this period to be found. In 

 the higher and later genera the process of acceleration or 

 earlier inheritance has pushed forward certain characters 

 until they appear in the protaspis, thus making it more and 

 more complex. 



