350 SYSTEMATIC PALEONTOLOGY 



downwards. In some the portion of the posterior margin outside of the 

 eye curves forwards, and brings the cheek angle to a position in front 

 of the eye. In others, it is behind the eye. The space between the eye 

 and the dorsal furrows is convex, and the eye itself seems to be rather 

 strongly protuberant or subconical. The movable cheek is subtriangular, 

 its width at the posterior margin about one and a half the distance of the 

 eye from the dorsal furrow, its length along the lower margin a little 

 greater than its posterior width. The anterior margin of the whole head 

 is uniformly rounded, with the exception of a slight concave curve just 

 outside of the suture. In some specimens in which .the front part of the 

 head is most abruptly bent down the middle portion of the front margin 

 is depressed convex or nearly straight. 



" Thorax with ten segments. Axis moderately convex, from a little 

 more than one-third to nearly one-half the width of the whole animal, a 

 little wider at the anterior than at the posterior segment ; the sides some- 

 times straight, and sometimes slightly curved outwards. On each side 

 of the axis there is a flat space between the side of the axis and the bend 

 of the pleurae. The width of this space is between one-third and one-half 

 the width of the axis. The pleurae are bent at the fulcra at an angle 

 which varies in different individuals, from 25 to 45, and at nearly 

 one-half their length from the side of the axis. 



" Pygidium usually a little shorter than the thorax ; varying from 

 moderately to rather strongly convex; the posterior margin broadly and 

 uniformly rounded; the anterior angles truncated nearly half the whole 

 length of the pygidium ; the straight sides formed by the truncation form- 

 ing an angle of from 40 to 60 with the longitudinal axis of the body. 

 The axis of the pygidium is well defined at the anterior margin by the 

 dorsal furrows, which die out at about one-third or one-half the length, 

 converging towards each other, and sometimes obscurely defining the 

 apex." Billings, 1865. 



Occurrence. MARTINSBURG SHALE (Sinuites bed). Chambersburg, 

 Pennsylvania. Trenton limestone of Ottawa, Canada, and many other 

 localities. 



Collection. IT. S. National Museum. 



