538 Dr. H. Woodward — M. Cambrian Fossils of the Rockies. 



The glabella is large, moderately convex, sides nearly straight, 

 very slightly expanded in front, reaching close to the anterior 

 margin of the cephalic shield, which forms a narrow raised rim. 

 There are four well-marked glabellar furrows, the hinder pair beicg 

 the largest, and running obliquely backwards, the third and four:h 

 pairs curve forwards towards the front of the glabella. 



The axis of the cervical (or occipital) ring is slightly indented an 

 each side on its anterior margin, and there is the raised base of 

 a central spine, directed backwards, as in other Onellidee, although 

 the spine itself is wanting. The side-lobes or pleurse of the cervical 

 ring are short and arched, and terminate laterally in a small stDut 

 spine directed outwards and backwards, and lying well within the 

 great lateral spines of the free cheek. The fixed cheek has a w:de, 

 semicircular expansion on each side of the glabella above the eye, 

 reaching from the cervical furrow behind to the fourth furrow on 

 the glabella in front, the eye itself making a long narrow rased 

 border to the fixed cheek. The facial suture nearly touches the 

 glabella in front, and then bends slightly outwards in a curved line 

 until it reaches the anterior margin of the head-shield. 



Fig. 5. — Zacanthoides [Olenoides) spinosus, Walcott, 1886. 



The head (in Zacanthoides) forms the broadest portion )f the 

 Trilobite, the spines of the free cheeks being directed ver^ much 

 outwards as well as backwards, and measuring 42 mm. across. 

 Their points do not reach further back than the second free somite. 

 There are nine free thoracic somites, the extremities of whi(h form 

 long, sharply recurved spines, overlapping each other and imreasing 

 in length from the head to the pygidium, the hindmost beiig more 

 than twice as long as the pleura itself; the pleura dimnishing 

 rapidly in breadth from before backwards as their spines increase 

 in length. 



The pygidium has four distinct coalesced annuli with a terminal 

 plate fringed by three small spines. The spines of che four 



