THE NATURAL HISTORY SURVEY. 275 



The dimensions of a small, nearly complete individual 

 are: total length 41.5 mm., greatest width between extremities 

 of genal spines 21.5 mm., length of head along median line n 

 mm., length of thorax 14 mm., length of pygidium 16.5 mm., 

 The dimensions of a nearly perfect head are: total length to 

 extremities of genal spines 43.5 mm., greatest width 44 mm., 

 length along median line 26 mm. The dimensions of a nearly 

 perfect pygidium are: length 42 mm., length of axis 17.5 mm., 

 length of caudal spine 24.5 mm., total width 30.5 mm. 



Remarks. This species is one of the commonest members 

 of the genus Dalmanites in the Chicago fauna. It is allied to 

 the British D. longicaudatus Murch., but differs from that 

 species in its flat caudal spine and in the less produced and 

 less mucronate anterior extension of the cephalon. The species 

 also resembles D, limnlurus (Green) from the New York Niagaran 

 faunas but has the caudal spine flattened and much more 

 elongate. 



The smaller, nearly complete specimen and the nearly 

 perfect pygidium illustrated on plate xxv, were collected by 

 Mr. L. H. Hyde. 



Locality. Chicago Drainage Canal near Lemont. 



Dalmanites illinoiensis, n. sp., pi. xxv, figs. 12. 



Description. Pygidium large, sub-triangular in outline, 

 wider than long, with a short caudal spine which is broad and 

 flat at its base becoming rapidly narrower and terminating in 

 a very acute point. The axis together with the segmented 

 portion of the pleural lobes, form a semi-elliptical, depressed 

 convex area surrounded laterally and posteriorly by a broad, 

 smooth, flattened marginal border which is produced pos- 

 teriorly into the caudal spine. The axis is depressed-con- 

 vex, sharply defined by dorsal furrows, its width at the anterior 

 margin of the pygidium is about two-ninths of the greatest, 

 width of the pygidium, it tapers posteriorly and terminates 

 in a bluntly rounded extremity at the edge of the flattened 

 marginal border, it is divided into fifteen segments by trans- 

 verse furrows which are deeply and sharply impressed laterally 

 but across the median portion they are shallower and less strongly 

 defined. The surface of the pleural lobes is flattened for about 

 one-half their width from the dorsal furrows and then gently 

 convex to the inner edge of the marginal border, they are di- 

 vided into eleven deeply grooved segments, the grooves ex- 



