232 CRUSTACEANS. 



with a projecting point on each side of the notch. Internal 

 surface concave ; external convex and smooth. 



Thorax about twice the length of the middle of the cephalic 

 shield, narrowing backward, and very strongly trilobate; me- 

 sial lobe as wide as the lateral, and distinctly more convex, 

 rounded or somewhat depressed on top and having its thirteen 

 segments usually a little thickened at their ends, but without 

 nodes. Lateral lobes separated from the middle one by distinct 

 furrows somewhat flattened on the inner third, and rounding 

 off more or less strongly to the lateral margins ; plurse extend- 

 ing straight outward for about one fourth to one-third of their 

 length, and then slightly deflected and curved backward to 

 their outer ends, which are rounded, compressed, somewhat 

 expanded, and provided with a thickened marginal ridge (not 

 seen externally), while the anterior face of their outer valves 

 are strongly flattened or beveled for sliding upon each other 

 in rolling up ; each with its longitudinal furrow well defined, 

 and placed so as to divide off, as it were, its anterior third, 

 though this is not seen more than half way out from their inner 

 ends, when the thorax is folded together. 



Pygidiam one-half to two-thirds the length of the middle 

 of the cephalic shield, wider than long, with a more or less 

 nearly sub-trigonal outline, the anterior margin, however, gen- 

 erally being so rounded as to impart a nearly transversely sub- 

 oval form to the general outline ; mesial lobe well defined, 

 depressed, convex, and extending very nearly to the posterior 

 margin, showing five or six segments, the last two being verj 

 faintly defined, while behind these there is space enough for 

 two or three more. Lateral lobes sloping or curving off more 

 or less rapidly, each with about five segments, only the ante- 

 rior one of which has a furrow like that of each of the pleurae. 

 Entire surface finely and evenly granular. (Meek.) 



Horizon and localities — Lower Silurian, Trenton lime- 

 stone: St. Louis county, Cape Girardeau. 



