CAMBRIAN TKILOBITES WALCOTT I9 



pygidium of Zacanthoidcs has ^11 the pleural segments extended as 

 spines directly across the border. 



The prominent differences between the two genera, then, are the 

 extension in adult individuals of the third or fourth segment of the 

 thorax in AlhcrtcUa, and the presence on the pygidium of one pair of 

 spines instead of many spines, as in Zacanthoidcs. 



The extension of the third segment of the thorax occurs in the 

 genera Oloicllus Hall and Mcsonacis Walcott,^ and the spinose 

 extension of the pleural elements of the pygidium occurs in Para- 

 boliiia Salter,- Hysfcrolcinis Moberg,^ and other genera, but these 

 other genera differ in so many other characters that it is unnecessary 

 to make comparisons between them and AlbcrtcUa. 



ALBERTELLA HELENA, new species 



Pr.ATE 2. Figures i-y 



Dorsal shield of niedium size ; with the exception of spines, longi- 

 tudinally elongate-ovate ; moderately convex. Cephalon semicircular 

 in outline, one-third the length of the dorsal shield ; marginal border 

 of medium width, slightly convex, delimited from the cheeks by a 

 sharp, shallow furrorw, and continued at the genal angles directly 

 into long, slender spines that extend outward and backward to a line 

 back of the union of the thorax and pygidium ; posterior border 

 narrow at the inner end next to the dorsal furrow, a little wider at 

 the facial sutures, and arching a little forward before merging into 

 the outer border at the genal spine ; the posterior border is delimited 

 from the cheeks by a narrow, shallow furrow that begins opposite 

 the center of the occipital segment, and, arching forward a very little, 

 passes into the furrow within the outer border. Cranidium convex, 

 subquadrangular in outline exclusive of the extension of its postero- 

 lateral limbs ; the latter are of medium width, with nearly one-half of 

 their area occupied by the posterior furrow and border. Fixed 

 cheeks at the palpebral lobe one-third the width of the glabella ; 

 posteriorly they merge into the postero-lateral limbs ; anteriorly thev 

 pass directly forward to the interborder furrow ; palpebral lobe 

 narrow, elongate, about three-fifths the length of the cephalon. 



M'Valcott, 1891, Tenth Ann. Kept. U. S. Geo!. Survey, pis. i.xx.xii, i,.\.\.\ni, 

 Lwxiv. L.xxxv, and i.xxxvii. 



- Moberg and Moller, i8g8, Meddelande fran Lunds Geo!. Faltklubb, No. 3 

 (Geol. Foren. i Stockholm Forhandlingar, Bd. xx, Haft 5, 1898, No. 187), pis. 

 xn and xni. 



^Moberg, 1898, INIeddelande fran Lunds Geol. Faltklubb, No. 5 (Geol. Foren. 

 i Stockholm Forhandlingar, Bd. xx, Haft 6, 1898, No. 188), pi. xvii, figs. 1-9. 



