70 Thirty-first Report on the State Museum. 



lobe convex, about two-thirds the width of the lateral lobes ; lateral lobes 

 strongly geniculate ; segments nearly transverse on the axial lobe ; pleural 

 grooves strongly marked. Pygidium subtriangular, strongly convex ; anterior 

 margin a little more than one-half as wide as the posterior margin of the 

 head ; axial lobe very prominent, tapering from the anterior margin to about 

 half the width at its posterior termination ; lateral lobes slope rapidly to the 

 margin ; fourteen segments are inqlicated on the axial, and eight on each of 

 the lateral lobes ; those on the lateral lobes extend nearly to the margin, and 

 have slight pleural grooves on the anterior three. 



Glabella slightly pustulose ; surface of thorax and pygidium apparently 

 smooth. 



This species is allied to 1). callicephalus Hall, D. breviceps Hall, and 

 D. Carleyi Meek. From the first it differs in the general outline, posterior 

 lateral spines of the head, shape of the glabellar lobes, smaller size of the 

 pygidium in proportion to the head, distinct character of the lateral segments 

 of the pygidium, and the absence of pleural grooves upon these segments, 

 with the exception of the first three. The difference in the glabella readily 

 distinguishes it from D. breviceps. The more rhombic frontal lobe, rudimen- 

 tary posterior lobes of the glabella, larger palpebral lobes, greater proximity 

 of the eyes to the glabella, separate it from D. Carleyi; and, if the pygidium 

 referred to the species, by its author, is the one belonging to it, the differences 

 with D. Carleyi are still greater. 



Formation and localities. Trenton Limestone. Two miles north of 

 Dunlith, 111. ; Clifton, Grant county, and Plattsville, Wisconsin. 



ILLiENUS, Dolman, 1826. 

 Ill^nus indeterminatus n. sp. 



General form, elongate elliptical, strongly convex. Head convex; broadly 

 rounded in front ; length two-thirds the width. Glabella moderately convex, 

 curves gently from the posterior margin to the anterior portion, where it 

 abruptly curves downward. The dorsal furrow is marked at its central por- 

 tion, by a strong oblong depression, anterior to a line with the eyes, and one- 

 quarter the distance between the eyes, from the palpebral lobe ; the anterior 

 extension extends obliquely forward toward the lateral margin terminating in 

 a rounded pit, which has a tubercle at the center ; posteriorly the furrow is 

 strongly defined, it passes obliquely backward nearly to the posterior mar- 

 gin, where it curves and running a short distance, cuts the margin on a line 

 with outer margin of the eye.* Eyes broken away with the movable cheeks ; 

 palpebral lobes indicate that they were of medium size, situated a short dis- 

 tance from the posterior margin. Facial sutures terminating on the posterior 

 margin outside a line with the outer margin of the eye ; anteriorly termina- 

 ting on a line with it. 



Thorax uniformly arched, not trilobed ; nine segments only can be counted ; 

 the crushing together of the segments may have forced one beneath the head. 



Pygidium parabolic, very convex, almost' tumid along the center; width a 

 little greater than the length,, broadly and uniformly rounded anteriorly ; an- 

 terior lateral angles depressed to admit of their passing beneath the posterior 

 segment of the thorax, when the animal was enrolled. 



Surface ; anterior margin of head with a band of strong lamellose striae 



meral course of the dorsal furrow, from the {interior termination, to the posterior 

 similar to tliat of the facial .suture in the yen us- ASAl'lll s. 



