88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



The largest shell has a length of 2.5 mm. ; width, 2.75 mm. 



Observations. — This neat little shell differs from Acrothcle stib- 

 sidiia (White) [1874, p. 6] in the absence of the surface characters 

 of that species and from A. spurri (see p. 86) by the more anterior 

 position of its beak and smooth surface. It belongs to the group of 

 Acrothele represented by A. horgholmcnsis (see p. 84). 



Formation and Locality. — Middle Cambrian : Calcareous shales 

 10 miles (16. op km.) south-southeast of Emigrant Peak, Silver Peak 

 quadrangle (U. S. G. S.), Esmeralda County, Nevada. 



Doubtfully: Middle Cambrian shaly limestones about 750 feet 

 (228.6 m.) above the top of the Cambrian quartzitic sandstones and 

 3,440 feet (1,048.5 m.) below the Upper Cambrian, about 9 miles 

 (14.48 km.) above the mouth of Blacksmith Fork and 16 miles 

 (25.75 km.) east of Hyrum, Cache County, Utah. 



ACROTHELE WOODWORTHI, new species 

 Plate 9, Figure ii 



Ventral valve transversely and irregularly oval in outline, convex, 

 with the apex a little back of the center of the valve ; pedicle open- 

 ing, as indicated by the matrix of the exterior, just back of the apex ; 

 a subtriangular, gently convex false area is obscurely outlined by 

 lines radiating from the apex to the posterior margin ; the posterior 

 margin is arched upward one-third or more of the distance from the 

 plane of the margin of the valve to the apex. 



Surface marked by concentric growth lines with fine strise between, 

 and a number of. low, rounded, more or less obscure, radiating ribs. 

 The shell is relatively thick and replaced by the calcareous matter of 

 the matrix. 



Observations. — This species is based on two specimens of the 

 ventral valve collected by Prof. J, B. Woodworth. The generic ref- 

 erence is not entirely satisfactory, as the pedicle opening has not been 

 clearly seen and the convex false area suggests the area of Micromi- 

 tra more than that of any Acrothele except A. spurri (see p. 86). 



Formation and Locality. — Lower Cambrian : Nahant lime- 

 stone. Pulpit Rock, Nahant, Essex County, Massachusetts. 



ACROTHELE YORKENSIS, new species 



Plate 9, Figure 10 



This species is represented in the fine clay shales by casts of the 

 exteriors of the valves and a few imperfect casts of the interior. 

 The shell is large for a species of this genus. Ventral valve mod- 



