CAMBRIAN BRACUWPODA—WALCOTT. 309 



OBOLUS (LINGULELLA) CHUARENSIS, new species. 



General form broadly ovate, almost subquadrate, with the ventral 

 valve obtusely acuminate, and the dorsal valve rounded subquadrate, 

 the posterior margin being broadly obtuse; convexity moderate, increas- 

 ing somewhat in the older shells. Surface of shcdl marked by rather 

 strong, concentric lines and stria' of growth, and very (ine, more or less 

 transverse and irregular, apparently imbricating stria' such as orna- 

 ment the surface of 0. (L.) ella, O. (L.) icillisij and O. {L.) rii(/h/j)lim. 

 Fine ra<liatiug stria- also ai)pear under a strong magnifying glass; 

 when the outer surface is exfoliated the inner layers of the shell show 

 traces of radiating stria*.; the inner surface is marked by pits or 

 puncta^, arranged in more or less irregular concentric lines; also tine 

 radiating stria'. The shell is strong and forn)ed of a thin outer layer 

 and several inner layers or lamelhc, those near the outer margin being 

 arranged obliquely to the outer surface. 



The only traces of the inteiior markings are those on the casts of 

 the dorsal valve. These show a short and rather broad area, strong 

 vascular sinuses, and traces of the interior lateral muscle scars. 



Ohserrations. — The character of the surface ornamentation and the 

 subquadrate form of the dorsal valve leads to comparison witli 0. (/>.) 

 willisi of the Middle Cambrian of the Southern Appalachian fauna, 

 and with 0. (/>.) ella of the Rocky Mountain fauna. The si)ecies differs 

 from those in being a thicker, stronger shell, and relatively shorter in 

 l)ro])ortion to its length. The surface is also of the same type as that 

 of O. (/y.) euglyphns, which occurs at the same horizon in the upper 

 portion of the Tonto sandstone, but not associated with it. It differs 

 from 0. {L.) euglypkus in being much shorter and broader in proportion 

 to its length. 



Formation and locality. — Middle Cambrian, ui)per layer of the 

 Tonto sandstone, at the head of Nunkoweap and Chuar valleys. Grand 

 Canyon of the Colorado, Arizona. 



Tyjte.—^o. 27310, (J.S.N.M. 



OBOLUS (LINGULELLA) DESIDERATUS, new species. 



Shell small, subovate, with the ventral valve obtusely acuminate, 

 and dorsal valve broadly ovate. Valves are strongly convex, with the 

 ventral valve fully as much so as the dorsal. There is a sliglit varia- 

 tion in the outline of the valves, some being slightly more rounded 

 posteriorly than others. 



The surface of the shell is marked by fine, concentric lines of growth, 

 and between them very fine, slightly irregular stria^; a few rather nar- 

 row indistinct undulations radiate from the umbo toward the, front and 

 lateral margins; when the outer shell is partially exfoliated tlie outer 

 surface of the inner layer is marked by very tine, indistiiict radiating 

 striae; there are a few traces of small, scattered pits or puncta; on the 



