298 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



ACROTRETA ATTENUATA Meek. 



Acrotreta atleimata Meek, Sixth Ann. Rep. U. S. Geol. Hur. Terr., 1873, p. 463. 

 A'Tofrt'ta gemma Walcott, Monog. U. S. Geol. 8nr. VIII, 1884, p. 17; Bull. 



V. S. Geol. Sur. No. 30, 1886, p. 98; Monog. U. S. Geol. Sur., XXXII, 1899. 



rt. 2, p. 449. 



The species is cliai'acterizod by its elevated ventral valve and narrow, 

 incurved false area. Some of the forms of A. idahoenms alta look 

 like A. ((tte)ti((ita, but the false area is different. ^1. l-utorgal has a 

 somewhat similar false area and elevation, but it is a less robust shell 

 and the apex of the ventral valve is nearer the posterior margin. The 

 reference to .1. (/etm/taiH explained under the description of the genus. 

 A shell with a distinctly marked false pedicle groove in the ventral 

 valve, occurs in the Bathynrli^cim >i^heelerl zone of the Middle Cambrian 

 of Utah that shows tine interiors of both valves. The interiors of the 

 ventral valve are nuich like those of A. hutoryal. 



Formation and loadlty. — Middle Cambrian. Flathead terrane. 

 Valley of East Gallatin River near Hillsdale. North of Gallatin 

 River. Base of limestone series resting on Flathead shales, Beaver 

 Creek, miles north of York, Big Belt Mountains, Montana, On 

 divide at head of Sheep Creek near the north end of Teton range, 

 Wyoming. South slopes of Marjum Pass, west of divide, House 

 range, Utah. Above Bathyiiriscus tnheelert zone 2i miles east of 

 Antelope Springs, House range, Utah. 



Upper Cambrian. In the Hamburg shale, a little south of the 

 Hamburg mine. Eureka district, Nevada. 



ACROTRETA ATTENUATA, variety.' 



A shell with a distinctly marked false area is separated as a variety 

 of .1. atti'iniafa. It is associated Avith the type specimens in the east 

 Gallatin Valley. 



ACROTRETA BISECTA Matthew. 



Acrohria halleyl Matthew? Trans. Roy. Soc. Canada, IX, 1892, j). 43, pi. xn, 



lig. 7(1. 

 Acrofreta Insecia Matthew, Bull. Nat. Hist. Soc. New Brunswick. IV, 1901, p. 



275, pi. V, figs. 5a-g; IV, 1902, pi, v, p. 394; pi. xvi, figs. 2, 2a-g. 

 Acrotirta xlpo Matthew, Bull. Nat. Hist. Soc. New Brunswick, IV, 1902, Ft. 5, 



]). 406, pi. XVIII, figs. 1 and 2. 

 Acrofrctu mpo Matthew, Geol. Sur. Canada, Rep. C'ambrian rocks. Cape Breton, 



1903, p. 185, pi. XVIII, figs. 1 and 2. 

 Acruhrt(( hi>feci(( Matthew, Geol. Sur. Canada, Rep. Cambrian rocks, Cape 



Breton, 1903, \>. 186, ])1. \i, figs. 5a-g. 



Nearly all the ventral valves of this species arfe more or less com- 

 j)ressed in the shale, thus decreasing the true elevation. Mr. Matthew 

 illustrates a pointed, high ventral valve, but does not state whether 

 tlie tio-ure is diaii'ramatic. Some of the casts in the shale indicate a 



