NO. 1299. CA MBRIAN BRA CHIOPODA— WALCOTT. 587 



acter of the limestone in which it is preserved, or it may be that it is 

 a little more calcareous than other species of the genus. 



Dr. G. F. Matthew describes and illustrates an Aerotreta from 

 Hastings Cove as Zi7inarssonia helti magna that is characterized b}- a 

 narrow median ridge in the brachial valve and other characters not 

 present in A. gracia. I found on Hanford Brook in the Par odo.i' ides 

 zone numerous examples of a species of Aerotreta that appear to be 

 identical with the form described b}- Mr. Matthew, and have referred 

 it as a variet}' of Aerotreta mglttalw. 



Formation and locality. — Middle Cambrian. Faradoxides zone, 

 Hastings Cove. Kennebecasis River. St. John County, New Brunswick. 



ACROTRETA IDAHOENSIS, new species. 



Aerotreta suhconica ^NIeek, Sixth Ann. Kept. U. S. Geol. Sur. Terr., 1873, p. 463. 

 Not Kutorga, 1S47. 



The general outline, form, and convexity of the two valves are so fully 

 shown by the illustrations for the monograph that detailed descriptions 

 will not be given. The material for illustrating the interior of the ped- 

 icle valve is limited, but one cast shows the position of the base of the 

 cast of the foraminal tube and the large main vascular sinuses. An 

 interior of abrachial valve presents characters rarelv seen in this g-enus. 

 The vascular canals arch out around the central muscle scars and then 

 bend in toward the median ridge, and again obliquely outward to form 

 the outer limit of a smooth, polished, lanceolate-shaped surface that 

 extends obliquely outward from the median ridge. The line that extends 

 from in front of the cardinal scars forward subparallel to the margin of 

 the shell and then bends abruptly in and forward is apparently the outer 

 boundary of a very broad, shallow, vascular sinus. Something of this 

 vsame character occurs in the brachial valve of Oholus [FinguJohidas) 

 sjjis.'^ifx. The cardinal scars are small and situated close to the narrow 

 area. A cast of the interior shows the central scars distinctly: also 

 the presence of a thickening of the shell in the vicinity of the cardinal 

 scars. 



Ohservations. — The external characters of this species suggest Aero- 

 treta eurvata., but the elevation and curvature of the pedicle valve are 

 not the same and the interior of the brachial valve is ver}^ distinct in 

 its specific characters. There are no interiors of the form from the 

 Black Hills, but the external characters appear to be the same. These 

 include the broad false area, with its slighth" marked groove, the posi- 

 tion of the apex, and the curvature of the false area. 



Formation and locality. — Middle Cambrian, Five miles southeast 

 of Malad City, Idaho. Northwest suburb of Deadwood, Black Hills, 

 South Dakota. South side of Gallatin Valley, in several sections of 

 Flathead and Gallatin terranes; also Crowfoot section, Gallatin Range, 

 Yellowstone National Park, Wvoming. 



