SYNOPSIS OF GENERA. 
xliii 
Diagnosis. Body generally small, elongate- 
ovate in outline. Cephalon semicircular, margin 
usually conspicuous and elevated; genal angles 
rounded or produced into spines. Glabella tapering 
anteriorly, scarcely reaching the frontal margin; 
lateral furrows conspicuous, faint or obsolete on 
the dorsal surface, distinct upon the internal sur¬ 
face. Eyes large, semilunate; lenses small, homocorneal. Facial sutures be¬ 
ginning on the posterior margin, just within the genal angles, and running from 
the eyes in nearly straight lines to the frontal margin, terminating on the edge 
of the doublure. Hypostoma elongate-rectangular, margins incurved and cen¬ 
trum very convex. Thorax longer than the cephalon, composed of from eight 
to ten segments, which are sulcate upon the pleurae, sharply angled at the 
fulcrum, and rounded at their extremities. Pygidium sub-semicircular. Axis 
arched and bearing from four to thirteen annulations; pleurae annulated; mar¬ 
ginal area broad and entire. 
The genus Proetus possesses a greater specific representation in the Devo¬ 
nian formations of North America than any other group of Trilobites, and 
appears to have attained its culmination of development both in species and 
individuals in the waters which deposited the limestones of the Upper Helder- 
berg group. But notwithstanding the abundance of specimens in these rocks, 
our knowledge of many of the species is imperfect on account of their usually 
fragmentary condition, and more complete material may eventually require the 
union of some of the forms described from detached parts of the animals, and 
which are now of necessity regarded as distinct species. In the higher Devonian 
formations the species are fewer, but the specimens are usually more complete 
and more satisfactory for purposes of description. 
The material which has been at hand for the study of the twenty-six Devo¬ 
nian species here described is fairly representative, and in some instances com¬ 
paratively abundant. All our observations point distinctly to the fact that the 
genus Proetus includes a well-defined group of Trilobites, subject to but little 
variability in essential characters, and that the numerous subdivsions of the 
