96 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



main vascular sinus on each side of the area; one cast shows traces 

 of the sinuses nearly to the front margin. The interior of the dorsal 

 valve is marked by a strong median septum or ridge that extends 

 from the front of a small cardinal area forward to nearly the ante- 

 rior margin in some examples. A large, oval cardinal muscle scar 

 occurs on each side of the median ridge near the posterior border of 

 the valve ; the central muscle scars are small, elongate oval, and 

 situated on the sides of the median ridge at about the posterior third 

 of the longitudinal axis of the shell ; the two antero-lateral muscle 

 scars are on the sides of the median ridge a little in advance of the 

 central scars. 



This is one of the large species of the genus. A dorsal valve 

 4 mm. in length has a width of 4.5 mm. The ventral valves average 

 from 4 to 4.5 mm. from the apex to the front margin. 



In size and outline this species may be compared with Acrotreta 

 depressa (Walcott) [1889, p. 441] and A. definita Walcott [1902, 

 p. 594] . It differs from both in its ventral valve being more elongate. 

 All the specimens of A. rudis are so flattened in the shale that com- 

 parison with uncompressed specimens is very difficult. 



Formation and Locality. — Middle Cambrian: (i) Rogersville 

 shale, just above the road, in the hill west of the school-house, j.5 

 miles (3.6^ km.) from Rogersville, on the road to MeUndy's Ferry; 

 and (2) Rogersville shale, road just east of Harlan's Knob, 4 miles 

 (6.44 km.) northeast of Rogersville; both in Hawkins County, Ten- 

 nessee. 



ACROTRETA ULRICHI, new species 



Plate 9, Figure 3 



This species is founded on a single specimen of a finely preserved 

 ventral valve. The outline of the aperture is nearly circular, except 

 for a short transverse portion beneath the false area ; surface moder- 

 ately convex, with the apex curving and ending beyond the posterior 

 margin, so that the minute foraminal aperture opens backward ; a 

 small false area is indicated by a slight incurving at the cardinal 

 angles ; the area is without traces of a median furrow. 



Surface of shell marked by very fine, concentric strise and lines 

 of growth. Length and width of aperture, 2 mm. ; convexity of 

 ventral valve, i mm. 



Observations. — This species is characterized by its curved umbo 

 and apex and overhanging false area. It most nearly resembles 

 A. curvata Walcott [1902, p. 584], from which it differs in being 

 less convex and in the form of its umbo and curved apex. 



