CAMBRIAN TRILOBITES — WALCOTT 4I 



Pygidium : mm. 



Length 2.9 



Width at union with thorax 6.0 



Observations. — This species is rather rare at Mount Stephen, 

 although 18 specimens were found in the collections of 1907. The 

 largest specimen of the dorsal shield has a length of 18 mm. The 

 strong triangular nodes on the pleural portion of the segments next 

 to the axial lobe and the nodes at the geniculation, combined with 

 the clearly defined furrows about them, give the thorax a very 

 striking ornamental effect that leads to giving the specific name 

 oniatus. This type of thoracic segment serves to distinguish the 

 species from all other species of the genus Bathyitriscns. The 

 associated B. rotundatus (Rominger)^ has quite a dift'erent pleural 

 segment, larger pygidium in proportion to the length of the dorsal 

 shield, and nine thoracic segments instead of the eight, as in B. 

 oniatiis. 



Another associated species, Bathyurisctts occidentalis (Matthew)." 

 has nine segments with an open pleural furrow, relatively smaller 

 pygidium, and larger free cheek. 



FoR.MATiON AND LOCALITY. — Middle Cambrian: Ogygopsis shale 

 of the Stephen formation, 2,400 feet (731.5 m.) above the Lower 

 Cambrian and 2,600 feet (792.5 m.) below the Upper Cambrian; 

 northwest slope of Mount Stephen, 3,000 feet (914.4 m.) above the 

 Kicking Horse River, above Field, on the Canadian Pacific Railway, 

 British Columbia, Canada. 



^ Lliiiboliimis rotundatus Roniinger, 1887, Proc. Acad. Nat. Sci. Philadelphia, 

 p. t6. 



" Dolichometopus occidentalis Matthew, 1899. Trans. Roy. Soc. Canada for 

 1899, 2d ser., vol. V, sec. 4, No. 2, p. 49. 



