396 PALAEONTOLOGY OF ILLINOIS. 



linear anterior continuation, and a less distinctly denned, 

 secondary transverse linear ridge. Ocular ridge narrow, but 

 distinct, its lateral divisions arching inwards behind the eyes, 

 and terminating posteriorly at the margin of the shield, nearly 

 opposite the middle of each lateral lobe of the abdomen, in a 

 (triangular) spine ? which is directed backwards, outwards, and 

 a little upwards ; anterior transverse division arching forwards 

 on each side, and curving backwards in the middle. Com- 

 pound eyes small, remote and located one at each anterolate- 

 ral angle of the crown-shaped central area, at points about one- 

 third the length of the shield from its anterior margin. Sim- 

 eyes, if they existed, unknown. 



Abdomen transversely suboval, being wider than long, and 

 nearly straight anteriorly, with lateral margins, rounding in 

 abruptly in front, and more gradually into the regularly 

 rounded posterior outline; generally rather more depressed 

 than the cephalo-thorax, particularly in front. Flattened late- 

 ral border rather narrow, and regularly scolloped between its 

 projecting marginal spines. Mesial lobe narrow, or of about 

 the same breadth as that of the cephalo-thorax, and near half 

 as broad as, and a little more elevated than, the lateral lobes; 

 segments well defined ; first and third each provided with a 

 small central tubercle; sixth as long as any three of the others, 

 rather abruptly narrowed and depressed behind, and sur- 

 mounted anteriorly by a large tubercle (or spine ?) . Lateral 

 lobes somewhat flattened on the inner half, and rounding down 

 rather abruptly to the flattened free borders on each side and 

 behind ; segments defined by distinct linear ridges, which are 

 separated by flattened spaces four or five times as wide; these 

 ridges extend obliquely outwards and a little backwards across 

 the lateral lobes and their flattened borders, beyond which 

 they are produced into slender mucronate spines, of nearly 

 equal length, curved obliquely backwards. 



Telson apparently nearly two-thirds as long as the abdomen, 



