16 PALEONTOLOGY OF THE EUREKA DISTRICT. 
the apex is not positively known, as its summit is broken off in all the 
specimens in the collection. 
The line from the apex to the margin is too strongly represented in the 
figure on plate ix. 
Formation and locality—Cambrian, Prospect Mountain Group; low 
down in the massive limestone beneath the Secret Cafion shale on the east 
slope of Prospect Mountain, Eureka District, Nevada. 
Genus ACROTRETA Kutorga. 
Mr. Davidson, in his Classification of the British Fossil Brachiopoda, 
gives an accurate generic description of the external form and character of 
the valves of Acrotreta, but not having specimens showing the interior of 
the valves, he left the genus doubtfull-; as a subgenus of Siphonotreta. 
The fortunate discovery of a large number of specimens of Acrotreta gemma 
Billings, in the passage-beds between the Cambrian and Silurian limestones 
of the Eureka District, affords material for the study of the interior char- 
acters of the smaller valve, and also some of those of the larger or conical 
valve. 
The interior of the larger valve shows a perforated, rounded, nipple- 
like projection extending down from the apex, plate 1, fig. 1/ This is well 
shown in the cast, fig. 1b. On each side a slight ridge extends obliquely 
forward. No other markings can be determined. In the smaller valve a 
strong median ridge extends from in front of a minute triangular space on 
the cardinal margin two-thirds the distance to the front margin of the shell. 
Just back of the center of the shell a small oval scar occurs on each side of 
the ridge, and near the cardinal margin another pair of larger crescentiform 
sears. A pair of lateral marginal scars are very faintly shown on two ex- 
amples. They are usually too indistinct to be recognized. 
Comparing this interior with that of the smaller valve of Siphonotreta 
unguiculata, as illustrated by Davidson (Geol. Mag. N. S., vol. iv, pl. ii, 
fig. 11, 1877), the differences between them are well marked, especially 
in the greater spreading out of the muscular impressions in Acrotreta. 
