NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES 121 



and but slightly defined from the frontal limb. A narrow transversely 

 lined doublure extends back beneath the frontal border. 



Thorax. — Posterior extremity of thorax imperfectly known, so 

 that the number of component segments cannot be definitely deter- 

 mined ; nine are preserved on one specimen. Axial lobe not so wide 

 as the flattened pleura, sharply defined both by the abrupt elevation 

 and the lateral furrows : annulation rather coarse. Pleural segments 

 slender, elongated, obtusely angulated at their extremities ; pleural 

 furrows linear and sharply incised. 



Pygidiunt. — Pygidium not preserved, but from the narrowing of 

 the thorax undoubtedly small. 



Dimensions.- — -Length of dorsal shield, 7.1 mm. ; greatest width 

 of dorsal shield exclusive of the free cheeks, 5.o±mm. ; length of 

 cranidium, 3.7 mm. ; length of glabella, 2.7 mm. 



Surface. — Apparently nearly smooth. 



Formation and locality. — Middle Cambrian: (55c) Spence shale 

 member of the Ute formation; about 50 feet (15.2 m.) above the 

 Brigham quartzite, and 2,755 ^^^t (839.7 m.) below the Upper Cam- 

 brian, in a ravine running up into Danish Flat from Mill Canyon, 

 about six miles (9.7 km.) west-southwest of Liberty and 15 miles 

 (24.2 km.) west of Montpelier, Bear Lake County, Idaho. 



VISTOIA, new genus 



Vistoia is proposed for a small trilobite from the lower^ portion 

 of the Middle Cambrian limestones near the southwestern base of 

 Robson Peak. The cranidium and thorax suggest Corynexochus 

 stephenensis Walcott," but the pygidium is more of the type of that 

 of Eodiscus punctatns (Salter) with the transverse furrows omitted 

 from the axial lobe. The presence of five thoracic segments also 

 seems to distinguish it from Corynexochus, which has from 7 to ii 

 segments. Eodiscus, as far as known, lias three thoracic segments. 



Genotype. — Vistoia prisca, new species. 



Dimensions. — The only entire specimen known has a length of 

 13 mm. 



Range. — The one known species occurs in the lower portion of 

 the Middle Cambrian of Robson Peak, British Columbia, Canada. 



Observations. — All that is known of the genus is included in the 

 description of the genotype. 



' Smithsonian Misc. Coll., Vol. 64, No. 5, 1916, pp. 324. 325, pl- 55, fig- S- 



