590 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



The outer surface is marked by fine concentric strise and lines of 

 growth. 



Observations. — This pretty species occurs attached to the surface of 

 siliceous nodules in association with Olenoides curticei,, Crepicephalus 

 texamis., Acrothele hdlula, and casts of Medusa. The outline of the 

 convexity of the valves and the posterior view of the pedicle valve is 

 most like that of A. attenuata., but the interior markings are quite 

 unlike. 



JBormatioii and, locality .—IsMx^dX^ Cambrian. Siliceous nodules in 

 Coosa shales, Coosa Valley, 3 miles south of Gadsden, and near Che- 

 pultepec on road near Wades Gap, Alabama. Rogers ville shale, 

 Rogersville, and north and south of the town, Tennessee. 



ACROTRETA MICROSCOPICA MISSOURIENSIS, new variety. 



The shell is uniform!}' larger than the Texas shells and the vertical 

 median line of the false area is more depressed and distinct. Some of 

 the specimens of the pedicle valves beautifully illustrate the effects of 

 compression. One shell is like that of Acrotreta attemiata^ and another 

 resembles the low form of A. ophirensis with the apex overhanging 

 the false area. The surface of the shells embedded in a very fine- 

 grained sandstone has a papillose surface resulting from the indenting 

 of the shell by the grains of sand, while those from limestone are 

 marked only b}'^ fine concentric stria?. 



Formation and locality. — Middle Cambrian, in sandstones of the 

 conglomerate series, St. Francois County, and in thin-bedded lime- 

 stones south-southwest of Potosi, Missouri. 



ACROTRETA MICROSCOPICA TETONENSIS, new variety. 



This neat little shell possesses the characters of A. microsco2)ica and 

 A. idahoensis, but differs from both by the projection of the apex of 

 the pedicle valve over the false area. All that is known of it is repre- 

 sented by th^ illustrations for the monograph. 



Formation and locality. — Middle Cambrian, divide at head of Sheep 

 Creek, north end of Teton Range, Wyoming. 



ACROTRETA MISER Billings. 



Obolella f miser Billings, Can. Nat. and Geol., new ser., VI, 1872, p. 470; Pal. 



Foss., II, 1874, Pt. 1, p. 69. 

 Linnarssonia misera Matthew, Trans. Roy. Soc. Can., Ill, 1886, p. 35, fig. 12. — 



Hall and Clarke, Pal. N. Y., VIII, 1892, Pt. 1, p. 108, pi. viii, figs. 35-37. 



The abundant material that I collected of this species in Newfound- 

 land gives illustrations that present its characters with such fullness 

 and detail that minute description is unnecessary. Numerous figures 

 of the pedicle valve .show variation in outline, also of the cast of the 

 apical callosity, cardinal scars, and base of main vascular sinuses. 

 Others well illustrate the interior of the brachial valve. 



