SILURIAN SPECIES. 21 



tlie ])Osterior marg-in between one-fourth and one-third the distance from the 

 lateral angles inward toward the glabella. Eyes rather depressed, slightly 

 arched outward, separated from each other by a space somewhat less than 

 half the entire breadth of the head, and placed less than their own length in 

 advance of the posterior margin, and about once and a half their length be- 

 hind the front margin of the head; visual surfaces narrow, and not showing 

 any lenses under a good magnifier. 



Thorax with its length bearing the proportions to that of the head of 

 79 to 52, and to its own breadth of 79 to 107, being very slightly wider 

 near the middle than in front, and narrowing posteriorly, Avith gently convex 

 lateral margins, from behind the middle to the pyg-idium. Axial lobe de- 

 pressed, narrow, or only about two-thirds the breadth of each lateral lobe 

 at its anterior end, and narrowing regularly with straight sides posteriorly; 

 segments thirteen, nearly or quite straight, and each with some appearance 

 of a small node or prominence at each end.* Lateral lobes depressed or 

 nearly flat ; pleurae almost transverse, or arching slightly backward to near 

 the extremities, which are abruptly pointed ; each with a well-defined fur- 

 row, which commences small near the anterior inner end, and widens and 

 deepens for about half-way out, and then narrows and becomes more shal- 

 low, so as to die out before reaching the lateral extremities. 



Pygidium subsemicircular, being rounded posteriorly, with a narrow, 

 slightly-flattened border, and somewhat rounded anterior lateral extremities; 

 length bearing to that of the thorax the proportions of 30 to 79, and to that 

 of the head of 30 to 52, with a breadth of not quite two-thirds that of the 

 head; axiallobe eqiialing more than two-thirds the length, narrow, depressed, 

 and showing more or less distinctly about five segments; lateral lobes much 

 depressed, nearly twice as wide at the anterior end as the middle one, each 

 with about three segments, which curve a little backward, and become 

 obsolete before passing- u]X)n the narrow, smooth border; segments each 

 provided with a comparatively large longitudinal furrow, corresponding to 

 those on the pleurae. 



* lu the specimens, tbese little prominences seem to be so very obscure as to 

 leave doubts of their real existence as nodes. They are much too distinctly defined in 

 our figure. 



