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SILURIAN TRILOBITES. 



one or two concentric lines, as if of growth, mark the surface, which is also scabrous, like 

 the general crust of the body. The axis of the thirteen body-rings is convex, but narrower 

 than the pleurae, and constantly tubercular on the sides. The pleurae are horizontal half- 

 way, and then strongly decurved ; their ends rounded posteriorly, and bent forward. 

 Fulcrum distant from the axis, about half-way from it near the head, — at one third, or 

 rather less, behind. Pleurae sharply furrowed, the forward or fulcral half somewhat 

 narrower than the posterior. Tail nearly semicircular, with the front angles truncated ; 

 evenly and gently convex, the axis not prominent, the sides decurved strongly towards 

 their edges. Axis not percurrent, narrow, conical, with seven rings and a terminal boss. 

 Lateral ribs flattened, separated by sharp, narrow furrows, starting at a wide angle from 

 the axis, and curved back on the sides, simple, or but rarely marked by a central line near 

 their ends, — not bifurcate, as in C. Blumenbachii. 



Junior. — The proportions of the axis to the sides, and the structure of the pleurae 



are similar ; but the glabella is more cylindrical, not widened below ; the tail is propor- 

 tionately smaller, has the axis wider and more convex, with fewer ribs ; and there are but 

 four distinct ribs on each side. 



Variations. — In some the axis is a little more prominent ; in others a greater or less 

 depression of the glabella occurs, and apparently the production forwards of the snout is 

 not always in the same degree. But these variations are within narrow limits, and our 

 species never seems to approach C. Blumenbachii in convexity, especially with regard to 

 the glabella and caudal axis. The front is constantly produced, the surface minutely 

 scabrous, not covered with scattered tubercles ; but this last character occurs in some 

 varieties of C. Blumenbachii, which is more variable than we formerly believed. 



This really beautiful species shows in the reduction of the glabella a tendency towards 

 the characters so strongly displayed in the Lower Silurian forms. In the projecting 

 buttress which stretches from the region of the eye towards the second lobe of the glabella, 

 an approach is made to the very curious C. camerata of Conrad, in which the processes from 

 behind the eye form projecting wing-like covers to the axal furrows. And in the slight 

 furrows of the arched lateral lobes of the tail, several foreign species are imitated. 



Localities. — Wenlock Shale, Burrington, Shropshire, abundant. Ludlow Rocks, 

 Underbarrow, Westmoreland. 



Foreign localities. — Clinton Group (May Hill Group). Hall's figure probably repre- 

 sents this species. 



