42 PEIMOEDIAL PERIOD. 



straio-lit, but arching' a little backward at their outer ends; extremities 

 abruptly pointed, the points directed obliquely backward, and also bent a 

 little horizontally outward, so as to produce the appearance of a slightly- 

 flattened, serrated border along each side of the thorax, each pleura hav- 

 ing a well-defined groove, which is widest and deepest about the middle, and 

 extends from the inner end to near the outer extremity, where it becomes 

 obsolete and ends at the flattened tip. 



Pygidium subsemicircular, comparatively small, only a little more than 

 one-third as long as the thorax, regularly rounded behind, where it has a 

 narrow, flattened border of nearly uhifonn width all around; axial lobe 

 ending at the marginal border ; segments indistinct, but five or six in num- 

 ber are recognizable in some specimens ; lateral lobes much depressed, 

 twice the width of the axial lobe at the anterior end of the pygidium ; seg- 

 ments indistinct, but may be distinguished by then- grooves, which are 

 deeper than those that mark the limits of each; grooves cm-ving backward, 

 and, like those of the pleurse, becoming obsolete upon reaching the flattened 

 marginal border. 



The whole surface is apj)arently smooth, except that there are some 

 faint indications of radiating striae upon the exterior portions of the head of 

 well-preserved specimens, discernible only by means of a lens. 



Length of the largest specimen in the collections, five centimeters ; 

 breadth of the same across the thorax, thirty-thi-ee millimeters. 



Although this species resembles an Olenus in general outline and aspect, 

 the possession of blunt and shortened, instead of extended and pointed, 

 plem-se, the presence of faint elevated lines between the eyes and glabella, 

 the radiating lines upon' the surface of the head, together with the other 

 characters above described and shown in the figures, leave no doubt as to 

 the propriety of referring it to the genus Gonocoryplic. The presence of 

 only thirteen thoracic segments instead of fom-teen (the number attributed 

 to the genus by Corda) is not regarded as of generic importance in this 

 case. 



A number of entire and more or less perfect specimens of this fine 

 Trihhite are contained in the collections, but they have all been a little flat- 

 tened by compression. The finer details of structure of most of them have 



