'rRII.OBlTKS OF IHK ChA/V LiMESI'OXK. 351 



Measurements. — A thorax and pygidiunl : length of thorax, i 2 mm. ; 

 pygidium, 13 mm. Width of thorax at anterior end, 21 mm. Width 

 of axis, 15 mm. Width of pygidium, 19 mm. 



A cephalon : length 10 mm.; width 14 mm. Width between eyes 

 1 1. 5 mm. Distance of back of eyes from ])Osterior margin, i mm. 



Another cephalon: length 30 mm.: width of cranidium, 44 mm. 



Another is 18 mm. long, cranidium, 22 mm. wide. 



Locality. — Very common at Chazy, Valcour Island, Plattsburgh 

 and Crown Point, New York.. 



Illaenus erastusi sp. nov. (Plate 13, figures 8-9.) 



Illiinus crassicauda / Hall, 1847, Paleontology of New ^'ork, volume i, page 24, 

 plate 4 bis, figure 13. Not llheiius crasshauda Hall, 1847, Paleontology of New 

 York, volume I, page 229, plate 60, figures 4<7-(/. 



Under the name llhemis crassicaiida ^ Hall figured a portion of the 

 cephalon of a comparatively large Iihemis from the Chazy. Speci- 

 mens answering to these figures and the description are not uncom- 

 mon, but as they do not belong to the species I Haunts crassicauda 

 Wahlenburg, it becomes necessary to propose a new specific name. 

 The species is not the same as the one figured by Hall on plate 60, 

 figures 4f and d of the first volume of the New York Paleontology, 

 nor does it agree with Fllicmis aniericaniis Billings, as Clarke considers 

 this second Ilhvniis crassicauda to do. 



All the specimens in our collection are of se])arated head and ab- 

 domen shields. No complete specimens have, as yet, been found. 



Description. 



Cephalon large, nearly semicircular, the genal angles obtusely 

 rounded but extending somewhat beliind the posterior margin of the 

 glabella. Glabella smooth, not differentiated. Dorsal furrows far 

 apart, joining the neck furrows, which are visible only back of the 

 eyes. The dorsal furrows run forward and approximately parallel, 

 for a short distance, then turn outward and fade out after reaching 

 three cjuarters of the distance to the anterior margin. In the course 

 of each, about halfway between its anterior end and the margin, is a 

 small jjit, and to this pit the dorsal furrow is connected by an almost 

 imperceptible depression in the surface. 



The eyes are large, situated well to the sides and far back. Free 

 cheeks large, extending a little back of the posterior margin of the 



