Contributions to Palaeontology. 



119 



In small specimens, there is a minute node near each 

 anterior angle. 



Thorax wide, the axis narrow, moderately convex, strongly 

 defined by the dorsal furrow, less than one-fourth the 

 entire width of the body : lateral lobes nearly flat, the 

 segments marked by a simple groove which begins on 

 the upper margin of the rib at the dorsal furrow, and, 

 becoming central at a point about half the length of the 

 rib, continues in the same direction, bending downwards 

 and extending into the slightly recurved extremity - 



Pygidium broad, more than once and a half as wide as 

 long: axis very prominent; the lateral lobes convex 

 near the axis; and, sloping rather suddenly downwards, 

 become depressed and flattened towards the margin. 

 Axis marked by four angulations, exclusive^of the lower 

 terminal one and the anterior articulating joint : lateral 

 lobes marked by four ribs which are deeply divided from 

 their origin, with a slight elevation close to the posterior 

 extremity of the axis and the anterior simple rib. The 

 extremities of the ribs terminate in a broadly expanded 

 limb, which is gently curved at the sides, and produced 

 at the postero-lateral angles into a short process, leaving 

 a wide gently curving posterior margin. 



The surface of the movable cheeks, of the caudal expan- 

 sion, and of the extremities of the thoracic segments, is 

 strongly striate. The lower surface of the pygidium is 

 more strongly marked than any other part. 



This species occurs in the higher beds of the formation, and 

 usually within fifty to one hundred feet below the Lower Magnesian 

 limestone. We also find a similar or identical species in the Magne- 

 sian limestone. 



In its extremes of size, this species has a wide range. The 

 smallest head which I have observed has a length of one-fourth of 

 an inch, including the frontal limb; while the largest one has a 

 length of more than two and a half inches, with a width between 

 the extremities of the posterior limbs of the fixed cheeks of more 



