330 EIGHTEENTH REPORT ON THE STATE CABINET. 



TRILOBITES. 



In the Annual Report on the Geology of Wisconsin for I860,* 1861, I 

 described two new species of Ill^enus, a new Calymene, and a species 

 of Dalmanites ;t and in the list of species appended to the Geology of 

 Wisconsin, Vol i, page 423, I noticed a species of Acidaspis [A. danai) 

 from the limestone of the Niagara group. 



A farther study of the collections in my possession, from the limestones 

 of Racine, Waukesha, and from near Milwaukee, with a few from Illinois, 

 has shown several other trilobites : among which the following may be 

 noticed. 



GENUS ILL^NUS (Dalman). 

 ILL^NUS ARMATUS (n. s.). 



Head short and gibbous, varying in its proportions from nearly twice as 

 wide as long, to length equalling two-thirds the width ; elevated in the 

 centre. The dorsal furrow makes a short rounded arch from the base of 

 the shield, and terminates in a perceptible impression, on a line with 

 the base of the eye and half way between it and the anterior margin. 

 Eyes very prominent, situated close to the posterior margin ; palpe- 

 bral lobe short ; furrow above it scarcely longer than the sinus below 

 the eye. Cheeks small, making not more than one-fourth .of the width 

 of the head, and the suture coming to the anterior margin a little 

 in advance of the eye. The posterior angles prolonged into short spines 

 which extend directly backwards. 



The pygidium in several specimens has the proportions of length and 

 breadth as 5 to 7, 6 to 8, and 7 to 10. 



The largest head identified as belonging to this species has a length of 

 about one inch. 



3 4 



Fig. 3. IllcBnus armatus. The head — natural size. 

 Fig. 4. Profile view of the same. 



* Published 1861. 



•f IllcBnun imperator of the Niagara group, I. taurus of the Trenton limestone, Caly- 

 mane mammillata of the Hudson-river group, and Dalmanites vigilans of the Niagara 

 group. 



2(j 



