CAMBRIAN FOSSILS FROM PIOCHE MOUNTAINS 299 

 Zacanthoides typicalis, Walcott 



(Plate III, Figs, 2, 2a, 2b, 2c, 2d, 2e, 2/) 



Olenoides typicalis, Walcott, 1886: Bulletin No. 30, U. S. Geological Survey, 

 p. 183, Plate 25. 



Zacanthoides typicalis, Walcott, 1888: American Journal of Science, Third 

 Series, Vol. XXXVI, p 165. 



This is the most widely distributed aAd characteristic fossil of the 

 Mid- Cambric horizon at Pioche. Several almost perfect specimens 

 were collected. The heads, free cheeks, and fragments of the body 

 are extremely abundant. The material at hand, however, differs in 

 a number of details from the type specimen as figured, but these dif- 

 ferences are not all constant, as there is considerable variation in the 

 collection. The writer has found no specimen that shows the exact 

 arrangement of the pleural lobes, as indicated in the figure of the 

 type. It will be seen by reference to this figure' that from posterior 

 to anterior each lobe overlaps the succeeding one. This is probably 

 a fault of the drawing, as all the specimens at hand show the oppo- 

 site condition. 



Of the entire collection of no less than twenty-five free cheeks and 

 the attached genal spines there is not one specimen comparable to 

 those in the figure to which reference has just been made. The most 

 common spine is much straighter, and makes a larger angle with the 

 axial lobe (Fig. 2). Several others, somewhat resembling those of the 

 type, show a decided outward flexure in the backward extension 

 (Fig. 2e). There are three or four nearly perfect specimens, in which 

 the genal spines pass back as far as the extremity of the pygidium. 

 In this form the spines on the postero-lateral limbs are also unusu- 

 ally long (Figs. 26, 2c). Another very spinose specimen shows the 

 presence of a long spine on the next to the last pleural segment 

 (Fig. 2d). These conditions are rather confusing, as otherwise the 

 specimens are all alike. The differences, however, can hardly be 

 considered sufficient to justify the making of a new species. It may 

 be that these variations in spinosity are simply sexual peculiarities, 

 as suggested by Barrande in the case of Paradoxides harlani. 



Location: Half Moon Mine, Chisholm Mine, Abe Lincoln Mine, 

 and at the upper water-tank above Pioche. 



^Bulletin No. jo, U. S. Geological Survey, Plate 25, Fig. 2. ■ 



