406 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



IllcEtiurus sp. Walcott/ igoS^Tsinania Walcott. 

 IllccHurus sp. Walcott,^ i9o8 = Undt. 

 Illcciiurus sp. Walcott,' 1908 = Asaphus .' 



ILL^NURUS QUADRATUS Hall 



Plate 45, figs. I, la-c 



Illcenurus quadratus Hall, 1863, Sixteenth Ann. Rep., N. Y. State Cab. Nat. 



Hist., p. 176, pi. 7, figs. 52-57. (Description and illustration of species.) 

 Illmiurus quadratus Hall, 1867, Trans. Albany Inst., Vol. 5, p. 168, pi. 2, figs. 



52-57. (Reprint of paper of 1863.) 

 Illcciiurus quadratus Chamberlin, 1883, Geol. Wisconsin, Vol. i, p. 130, figs. 



16 L-P. (Reproduces figures of Hall.) 



To Hall's detailed description of the dismembered dorsal shield 

 of this species I am now able to add that the thorax has 1 1 segments 

 and that the pygidium is much smaller proportionally than the 

 cephalon. 



Formation and locality. — Upper Cambrian: (78) St. Lawrence 

 formation; Osceola, Polk County, and (85) Prairie du Sac, Sauk 

 County, Wisconsin. 



(113) La Grange Mountain or Barn Bluff, near Red Wing, Good- 

 hue County, Minnesota. 



Family Mesonacid^e Walcott. 1891 

 Genus MESONACIS Walcott, 1885 

 See Mesonacis Walcott, 1910, Smithsonian Misc. Coll., Vol. 53, p. 261. 

 MESONACIS GILBERTI (Meek) 



Plate 45, fig. 3 



Olenellus gilberti Meek, 1874. For synonymy see Smithsonian Misc. Coll., 

 Vol. 53, 1910, pp. 324, 325. 



The discovery of a large, fine specimen of this species by Mr. 

 Edward Sampson, of Philadelphia, affords the data upon which to 

 change the generic reference of the species gilberti from Olenellus 

 to Mesonacis. The specimen is almost a perfect dorsal shield 20 cm. 

 in length. On the fifteenth thoracic segment a long median spine is 

 attached that extends back over seven or eight small segments and a 

 small plate-like pygidium. 



This species appears to be represented by the same variations in 

 both Nevada and British Columbia, Canada. These include variation 



^ Smithsonian Misc. Coll., Vol. 53, 1908, p. 175, la. 

 " Idem, Vol. 53, 1908, p. 177, 2a. 

 ' Idem, Vol. 53, 1908, p. 192, i. 



