ACROTRETID^. 711 



Dorsal valve slightly convex; apex marginal; surface marked by a rather broad, shallow- 

 median depression that begins on the umbo and gradually widens toward the front margin. 

 The interior of the dorsal valve has a strong median ridge extending from the posterior margin 

 two-thirds the distance toward the front. A small, elevated cardinal scar occurs on each 

 side of the median ridge a little in advance of the posterior margin. Main vascular sinuses 

 rather strong; they start beneath the apex and extend forward a short distance from the outer 

 lateral margin of the shell. 



Surface marked by fine concentric strise and lines of growth. 



Ohservations. — This shell in size and general form is closely related to Acrotreta microscojnca 

 (Shumard) (PI. LXVII) of the Middle Cambrian fauna of the United States. 



The specific name is derived from Shantung, China. 



Formation and locality. — Middle Cambrian: (C37) Upper part of the Kichou limestone o in dense black lime- 

 stone nodules in green-gray shales 10 feet (3 m.) below the base of the cliff limestone, 8 miles (12.8 km.) south of Ting- 

 hianghien; and (C71) massive cliff -making limestone in the central portion of the Kichou formation [Willis and 

 Blackwelder, 1907, pp. 139 and 145 (2d list of fossils)], 4 miles (6.4 km.) southwest of Tungyu, Shansi, China. 



(CI) Lower shale member of the Kiulung gi-oup [Blackwelder, 1907a, pp. 37 and 40 (part of the 3d list of fossils), 

 and fig. 10 (bed 4), p. 38], 2 miles (3.2 km.) south of Yenchuang; and (C62) earthy layer in the middle limestone member 

 of the Kiulung group [Blackwelder, 1907a, pp. 37 and 40 (last list of fossils), and fig-. 10 {base of bed 7), p. 38], '2.5 miles 

 (4 km.) south of Yenchuang, on the north-northeast spur of Hulushan; both in the Sintai district, Shantung, China. 



A shell apparently identical with Acrotreta slmntungensis was collected at the following 

 locality : 



Middle Cambrian: (C32) A fine-grained, bluish-black limestone bowlder believed to have come from the lower 

 part of the Kisinling limestone [Blackwelder, 1907b, p. 272], collected in river drift 1 mile (1.6 km.) south of Chon- 

 pinghien, on Nankiang River, southern Shensi, China. 



AOROTEETA SIGNALIS Walcott. 



Plate LXIX, figure 4. 



Acrotreta signalis Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 599. (Described and discussed as below as a new 

 species.) 



■ Ventral valve transverse, broadly ovate ; height about one-third the length ; apex directed 

 backward on a line with the posterior margin, which is transverse and slightly undulated at the 

 center by the median furrow of the false area; apex with a minute apical foramen which is 

 directed backward. Surface marked by fine concentric striaj and lines of growth that at the 

 median furrow of the false area curve toward the posterior margin. 



Shell small, 1.5 mm. long by about 1.75 mm. in width. Shell built up of several very 

 thin layers or lamellae. 



Ohservations. — In all of the extensive collections made from the Cambrian sandstones of 

 the upper Mississippi Valley, only two ventral valves of any species of Acrotreta have been 

 found, as far as known to me. These occur in association with Lingulella (Lingulepis) acuminata 

 (Conrad) and the shells have the color and appearance of those of that species, shiny light gray 

 to white, the original coloring matter having been leached out. Only the exterior of the ventral 

 valve is laiown. This compared with A. microscoinca (Shumard) (PI. LXVII) shows a less 

 elevated ventral valve and a less distinct false area. It also has a marked median groove on the 

 false area. It recalls A. idaTioensis Walcott, but differs in being more depressed and in having 

 a stronger groove on a narrower false area. 



FoEMATioN AND LOCALITY. — Middle Cambrian: (328e") "St. Croix sandstone" at St. Croix Falls, Polk County, 

 Wisconsin. 



Acrotreta socialis von Seebach. 



Plate LXXIII, figures 3, 3a-c, 4, 4a-e. . 



Acrotreta socialis von Seebach [not Linnaesson], 1865, Zeitschr. Deutsch. geol. Gesell. for 1865, Bd. 17, p. 341, PI. 

 Villa, figs. 1^. (Described and discussed in German, as a new species. Figs. 1^ are reproduced in this mono- 

 graph, PI. LXXIII, figs. 3, 3a-c, respectively.) 



a The fossils from this locality are not listed, but the presence of Cambrian strata at the locality is mentioned by Willis and Blackwelder 

 [1907, p. 146]. 



