CAMBRIAN TRILOBITES — WALCOTT 29 



later stages of growth of this species and a character persistent in 

 Alhertella helena, which occurs over 2,000 feet (609.6 m.) lower 

 than the horizon of Zacanthoides spinosus (Rominger) in the 

 Cambrian section of the Canadian Rocky Mountains. It also occurs 

 in the adult forms of Mesonacis vermontana^ and other trilobites of 

 the Olciicllus fauna. A specimen of the entire dorsal shield 3.2 mm. 

 in length has the same widening of the glabella toward the front 

 as the smaller .specimen, but the base of the palpebral lobes have 

 drawn in toward the glabella, and the glabella has extended forward 

 beyond the anterior extremities of the palpebral lobes ; the thorax 

 has only adult characters, except that the third segment appears to 

 have on one side a stronger terminal spine, and there are but seven 

 segments ; the spines on the border of the pygidium are short, and 

 but four can be seen on each side. Specimens 8 mm. in length have 

 all adult characters in the cephalon and thorax, with the exception 

 of the terminal spines of the pygidium, which are shorter and less 

 clearly defined at the crossing of the border. 



Observations. — This species occurs abundantly in Idaho. When 

 collecting it I thought it to be Zacanthoides typicaUs,' but on direct 

 comparison with that species it was found to differ in having the 

 posterior end of the palpebral lobe nearer the glabella ; the glabella 

 proportionally narrower in front,, and larger antero-lateral parts of 

 the fixed cheek; a broader thoracic axis in proportion to the pleural 

 lobes ; a long median spine on the fifth instead of seventh segment ; 

 a larger pygidium, with broader pleural lobes, more rings on the 

 axis, and more terminal spines on the pygidium. It is found to 

 differ from Zacanthoides spinosus (Walcott)'' in having the gla- 

 bella less expanded toward the front; palpebral lobes nearer the 

 glabella at their posterior end ; smaller antero-lateral parts of the 

 fixed cheek ; absence of a strong occipital spine ; in the thorax it 

 dififers in having a long median spine on the central axis at the fifth 

 segment instead of the seventh, and the axial lobe is proportionally 

 wider; the pygidium differs in having four rings on the axis in- 

 stead of three ; the axial lobe is proportionally longer, and the spines 

 on the pygidium differ in details of shape and number. The three 

 species occur at the same relative geological horizon, but are widely 

 separated. Z. typicalis occurs at Pioche, Nevada, 350 miles south- 

 southwest of the locality of Z. idahoensis at Spence Gulch, 15 miles 



'Walcott, 1891, Tenth Ann. Rept. U. S. Geol. Survey, pi. lxxxvii, fig. la; 

 see also pis. lxxxiv and lxxxv. 



- Walcott 1886, Bull. U. S. Geol. Survey, No. 30, p. 183. 



'Walcott, Mon. U. S. Geol. Survey, vol. viii, 1884, p. 63; and Bull. U. S. 

 Geol. Survey, No. 30, 1886. p. 184. 



