674 CAMBRIAN BEACHIOPODA. 



"Acrotreta? costata" Davidson [1883, p. 213] from the Llandeilo of Scotland does not appear 

 to be an Acrotreta. The description and figures suggest a genus allied to Cyrtia Hall and Clarke 

 [1894, p. 40] of the Silurian. 



The surface characters of Acrotreta have been considered to be very simple and confined 

 to concentric lines and striae of growth and obscure radiating striae. This study of the species 

 has resulted in the discovery that five species, A. hisecta Matthew, A. gemmula Matthew, A. 

 injlata (Matthew), A. primseva Walcott, and J., sahrinse (Callaway), have a surface much like 

 that of Oholus (Westonia) ella (Hall and Whitfield) (PI. XL VII). Mention has been made of 

 the cancellated and spinose species, and those with undulating concentric striae. It is evident 

 that Acrotreta, like AcrotJiele and Micromitra, has varied surface characters that are of specific 

 but not generic value. 



The false area, with or without its vertical median groove, is a variable character. The 

 area and median groove are well developed in the type A. suhconica Kutorga (PL LXXIII, 

 figs. 2g, 2h) and in the species A. socialis v. Seebach (PI. LXXIII, figs. 4', 4a'). On A. attenuata 

 Meek a narrow sharp furrow is present, but the false area is scarcely defiaed (PI. LXIV, figs. 

 lb", Ic", Id"). On A. idahoensis Walcott the false area is clearly defined but there is scarcely 

 a trace of the median furrow (PI. LXV, figs. 1" , la"-c"). On A. microscopica (Shumard) (PI. 

 LXVII, figs, la'-c') there is only a faint false area and no median groove. 



The vascular markings, muscle scars, and apical callosity of the ventral valve vary in 

 relative size, position, and appearance, but they all have a common general character and 

 position. Usually the markings on the interiors of the valves are very faint, and often nothing 

 can be seen but the apical callosity and. the base of the main vascular sinuses in the ventral 

 valve, and the median ridge in the dorsal valve. It would require many pages on which to 

 record the variations, and. as they are illustrated by the figures on the plates I will not attempt it. 



Geographic distribution. — The httle shell of Acrotreta occurs in every country where any 

 considerable development of the Cambrian fauna is known. The exceptions are northern 

 India, Australia, and where the Lower Cambrian fauna predominates. The general distribu- 

 tion is given on pages 105-106. 



Stratigrapliic distribution. — Three species and one variety are known from the Lower 

 Cambrian, one o^ \Yhich, A. primseva Walcott (PL LXIX, figs. 1, la-f), is one of the largest 

 species of the genus. From the Middle Cambrian tliirty-three species and ten varieties are 

 Imown; they range from the base of the terrane to its summit, and two, A. idahoensis Walcott 

 and its variety alta and A . pyxidicula Wliite, pass up into the Upper Cambrian, and the first 

 species and its variety alta continue on up into the base of the Ordovician in Nevada. In the 

 Upper Cambrian twenty-one species and thr,ee varieties are known and in the passage beds above 

 seven species. In the Lower Ordovician seven species and one variety occur, of which one 

 species and one variety also occur in the Cambrian. The stratigraphic range of the species is 

 given on pages 105-106. 



Acrotreta argenta Walcott. 



Plate LXVII, figures 5, 5a-g. 

 Acrotreta argenta Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 580-581. (Described as below as a new species.) 



The material representing this species is more or less crushed and broken, but a few speci- 

 mens show the general form and characters. It belongs to the Acrotreta idahoensis Walcott 

 group in having a wide false area, strongly marked apical callosity, and pedicle opening. The 

 thickening in the posterior portion of the dorsal valve is also characteristic of the two species. 



The cast of the interior of a ventral valve (PL LXVII, fig. 5b) reveals what I have searched 

 for unsuccessfully in hundreds if not thousands of specimens of this genus. I refer to the 

 visceral area with the position of the point of attachment of the central and lateral muscles on 

 each side of it, essentially as in OboleUa and Oholus (PL VII). 



Formation and locality. — Upper Cambrian: (7x and 7y) Limestone of the Emigrant formation [Turner, 1902, 

 p. 265], about 2.5 miles (4 km.) southeast of Emigrant Pass; (8b') shales of the Emigrant formation {Turner, 1902, p. 265], 



