182 STUDIES IN EVOLUTION 



section, and are important in a phylogenetic study of the 

 trilobites. The main features of the cephalon in the simple 

 protaspis forms of Solenopleura, Liostracus, and Ptychoparia 

 are retained to maturity in such genera as Carausia and 

 Acontheus, which have the glabella expanded in front, join- 

 ing and forming the anterior margin. They are also without 

 eyes or eye-line. Ctenocephalus retains the archaic glabella 

 nearly to maturity, and likewise shows eye-lines and the 

 beginnings of the free-cheeks (larval Sao'). Conocoryphe and 

 Ptychoparia are still further advanced in having the glabella 

 rounded in front, and terminated within the margin (larva of 

 Triarihrws). These facts and others of a similar nature show 

 that there are characters appearing in the adults of later and 

 higher genera, which successively make their appearance in 

 the protaspis stage, sometimes to the exclusion or modifica- 

 tion of structures present in the most primitive larvse. Thus 

 the larvae of Dalmanites or Proetus, with their prominent 

 eyes, and glabella distinctly terminated and rounded in front, 

 have characters which do not appear in the larval stages of 

 ancient genera, but which may appear in their adult stages. 

 Evidently such modifications have been acquired by the 

 action of the law of earlier inheritance or tachygenesis. 

 Altogether it seems that we have represented on Plates III 

 and IV a progressive series of first larval stages in exact 

 correlation with adult forms, the latter also constituting a 

 progressive series, structurally and geologically. 



A summary of the features added to the dorsal shield of 

 the anaprotaspis stage of acceleration during the evolution 

 of the class, from the simpler forms of Cambrian times to the 

 later and more highly differentiated Dalmanites, Proetus, and 

 Acidaspis, would include: the free-cheeks; the eyes; the 

 more strongly lobed glabella, rounded in front ; the transient 

 eye-line ; the genal angles ; and the ornaments of the test. 



These additions, as may be seen by reference to Plates III 

 and IV, considerably complicate and modify the primitive 

 protaspis, but, as previously mentioned, it does not lose any 

 of its essential structures. Besides, it is possible to trace 



