592 PROCEEDINGS OF THE NATIONAL MUSEUM. vol, xxv. 



preserved cast of the interior of the valve. The interior of the brachial 

 valve shows great variation in the size and length of the median ridge, 

 and cardinal and central scars. These characters are fully shown in 

 the numerous illustrations of the interior and casts of the interior of 

 the brachial valve. 



Oh.servations. — This species is most closely related to Acrotreta 

 currata. It differs in the pedicle valve being larger, in the apex 

 being less extended over the false area, and in the form of the median 

 furrow, which is like that of A. alternata. Although the shells occur 

 in a compact, unaltered limestone, there is more or less distortion in 

 the outline of both valves. This, taken in connection with the varia- 

 tions in form and size of the vascular markings, muscle scars, and 

 median ridge of the brachial valve, might serve to discriminate several 

 so-called varieties, but I do not think that any good purpose would 

 be subserved thereby. 



Formation, and locaUt)/. — Middle Cambrian, in limestone above the 

 basal quartzite, Oi)hir City, Utah. 



ACROTRETA OPHIRENSIS RUGOSUS, new variety. 



This form is associated with A. opJuvensls in considerable numbers. 

 It is characterized bv strong, rugose growth lines and thicker shell. 



ACROTRETA OVALIS, new species. 



In the material representing A. gemma received from the Geolog- 

 ical Survey of Canada there was one pedicle valve embedded in a 

 smooth, dove-colored limestone from Point Levis. In the oval outline 

 of the rim of the shell, less elevated apex, and less well-defined false 

 area this specimen differs materially from A. gemma. It is more of 

 the A. socialis type than any of the Atlantic province forms, and of 

 A. microsGojnca of the interior continental species. 



Formation and locality. — Lower Ordovician, Limestone No. 1 of 

 Mr. Billings's Point Levis section. Province of Quebec, Canada. 



ACROTRETA PARVULA Wallerius. 



OboleUa parvula Wallerius, 1895. Cndersokningar ofver zonen med Agnof^tns 

 la?vigatusi. Vestergotland. Akad. Afhandl. Lund., p. 65, pL, fig. 9. 



Shell minute, subcircular. Pedicle valve relatively depressed, the 

 highest point being at the umbo, from which there is a slight down- 

 ward curvature .to the apex. False area low and about vertical, the 

 apex being on a line with the posterior margin. Brachial valve slightly 

 convex; beak marginal. Surface marked by fine concentric stride. 



This minute species occurs with Agnostics Isevigatus in the upper 

 portion of the Middle Cambrian. It is of the Acrotreta sagittalis type, 

 but does not appear to be identical with the young of that species. 



Formation and locality. — Middle Cambrian. Dark-brown limestone 

 at Borgholm. Oland, Sweden, as collected by M. Schmalensee. Dr. 

 Wallerius gives Gudhem and Djupadal. Vestergotland, Sweden. 



