180 The Americjin Geohxjlst. September, 1895 
A number of genera ))resent acUilt charaeters, which agree 
closely with some of the larval features noticed in this section, 
and are important in a phylogenetic study of the trilobites. 
The main features of the cephalon in the simple protaspis 
forms of SoJenopleiira, Liontrdciis and Pi tjcJiofxirid ^ are re- 
tained to maturity in such genera as Ctaudisia and Aconfheus, 
which have the glabella expanded in front, joining and form- 
ing the anterior margin. They are also without eyes or eye- 
line. (Jtenocephahis retains the archaic glabella nearlj^ to ma- 
turity, and likewise shows eye-lines and the beginnings of the 
free-cheeks (larval tSao). Conocori/phe and Pfi/r/io/Jdria are 
still further advanced in having the glabella rounded in front, 
and terminated wuthin the margin (larva of Tri(irfhrus). 
These facts and others of a similar nature show that there are 
characters appearing in the adults of later and higher gen- 
era, which successively make their appearance in the protaspis 
stage, sometimes to the exclusion or modification of structures 
present in the most primitive larva. Thus the larvie of Dal- 
maiiife.s or Proetus^ with their prominent eyes, and glabella 
distinctly terminated and rounded in front, have characters 
which do not ap])ear 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 VIII and IX 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, Pro'etiis and Acid- 
a.sy^ As", 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 VIII 
and IX, considerably complicate and modify the primitive 
protaspis, but, as previously mentioned, it does not lose anj^ 
of its essential structures. Besides, it is possible to trace the 
origin and significance of the acquired characters, and thus to 
assign to each its true value. 
