PRIMORDIAL PERIOD. 45 



posterior margins bordered by a narrow, continuous, slightly-raised rim, 

 that of the external margin being* placed a little within its edge, that of 

 the posterior margin continuous across the occipital lobe of the glabella; 

 the postero-lateral angles produced into slender spines, which are terete, 

 very slightly incurved, not much widened at their bases, and in our sj^eci- 

 mens are about equal in length to one-fourth of the transverse diameter 

 of the head at its base. The posterior margin of the head, near each 

 postero-lateral angle, bends abruptly forward a little, forming a kind of 

 notch or small retreating angle with the backward-pi'ojecting spine, and 

 also giving the outer corner of the movable cheek the appearance of being 

 rounded. Eyes narrow, broadly arching outward, nairowness and con- 

 vexity of curve both increasing posteriorly; their outer margins nearly 

 equidistant from the center and the outer margins of the head, which dis- 

 tance is also about equal to the length of the eye. Glabella distinctly 

 lobed, the fuiTOws extending nearly or quite to its center; frontal lobe 

 much larger than the others, subcircular in outline, prominent, tumid, 

 regularly rounded in front, and a little wider than the remainder of the 

 glabella; anterior, middle, posterior, and occipital lobes of nearly uniform 

 size. 



The remainder of the body is unknown, vnth the exception of single 

 specimens each of the long- third pleura of the right side, and one of the 

 others, probably belonging behind the third segment. The long one is 

 broadly and somewhat deeply grooved upon the surface of its inner por- 

 tion, Avhere the anterior edge of the groove is elevated, the groove becom- 

 ing shallower and narrower upon the outer portion, and finally disappearing 

 toward the point of the pleura; outer portion of the pleura bending 

 strongly but not very abruptly backward, and ending in a strong, flattened, 

 spine-like point. The other pleura mentioned is grooved like the long one, 

 but the anterior edge of its inner portion is not turned upward, as it is in 

 the other. 



The specimens of the collection are all imperfect, consisting only of 

 flattened impressions in shale. Consequently, the character of the crust, 

 the surface-markings or ornamentation, if any existed, and the original 

 convexity of the head are all unknown; but there are indications that the 



