SYNOPSIS OF GENERA. xliii 



Diagnosis. Body generally small, elongate- 

 ovate in outline. Cephahm 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, homoeorneal. Facial sutures lie- 

 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. Pijgidium sub-semicircular. Axis 

 arched and bearing from four to thirteen annulations; pleurae annulated ; mar- 

 ginal area broad and entire. 



The genus Prottus possesses a greater specific representation in the Devo- 

 nian formations of North America than any other group of TrilobUes, 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 imjjerfect 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 Proitus includes a well-defined group of Trilobites, subject to but little 

 variability in essential characters, and that the numerous subdivsions of the 



