NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES I39 



DESCRIPTION OF PLATE 20 



PAGE 



Briscoia sinclairensis Walcott ■■ 75 



Fig. I. (Natural size.) Dorsal view of a fairly complete cranidium. 

 U. S. Nat. Mus., Cat. No. 70301. 



2. (Natural size.) Front portion of a large cranidium. U. S. Nat. 



Mus., Cat. No. 70302. 



3. (Natural size.) Associated free cheek assigned to the species. 



U. S. Nat. Mus., Cat. No. 70303. 



4. 5. (Natural size.) Dorsal and side views of a large pygidium, 



illustrating the course of the pleural furrows. U. S. Nat. 

 Mus., Cat. No. 70304. 

 6, 7. (Natural size.) Side and dorsal views of an associated 

 hypostoma referred to the species. U. S. Nat. Mus., Cat. 

 - No. 70305. 



8. (Natural size.) An associated free cheek preserving most of 



the genal spine. U. S. Nat. Mus., Cat. No. 70306. 



9. (Natural size.) Cast of a fairly complete pygidium. U. S. Nat. 



Mus., Cat. No. 70307. 

 10. (Natural size.) Slab with free cheek, thoracic segment and por- 

 tion of a pygidium. Note surface ornamentation. U. S. Nat. 

 Mus., Cat. No. 70308. 



Matrix, limestone. 



The specimens represented by figs, i to 7 are from Lower Ozar- 

 kian, Mons formation. (Loc. i6t') : Sinclair Canyon, British 

 Columbia, and figs. 8 to 10 are from Lower Ozarkian, Mons forma- 

 tion. (Loc. 64n) : Near base of le of field section. Glacier Lake 

 Section, Alberta, Canada. 



