696 CAMBRIAN BRACHIOPODA. 



dorsal valve has a strong median depression which starts on the umbo and widens out rapidly 

 toward the front. 



The surface of the shell is marked by concentric ridges and lines of growth and very fine 

 concentric striae. 



Observations. — This is one of the Acrotreta idahoensis forms of the genus. It differs from 

 other described forms in the strong median depression of the dorsal valve. 



The specific name is derived from Mount Nebo Canyon, the type locality. 

 Formation and locality. — Middle Cambrian: (14t) Limestone lying on slope between the Cambrian quartzite 

 and the massive blue limestone 100 feet (30.5 m.) above, in Mount Nebo Canyon, 3 miles (4.8 km.) southeast of Mona, 

 Juab County, Utah. 



AcROTKETA NiCHOLSONi Davidson. 



Plate LXXIII, figures-lrla-'*:, 6, 6a-b.' 

 Acrotretaf nicholsoni Davidson, 1868, Geol. Mag., vol. 5, pp. 313-314, PL XVT, figs. 14-16. (Described and discussed 



as a new species; see below for copy of description. Figs. 14, 15, and 16 are reproduced in this monograph, 



PI. LXXIII, figs. 1, Ic, and Id, respectively.) 

 Acrotreta ? nicholsoni Davidson, 1871, British Fossil Brachiopoda, vol. 3, pt. 7, No. 4, pp. 343-344, PI. XLIX, figs. 36-40. 



(Described and discussed. Figs. 36, 37, and 40 are copied from figs. 15, 14, and 16, respectively, of the preceding 



reference. Figs. 38 and 39 are reproduced in this monograph, PI. LXXIII, figs, la and lb, respectively.) 

 Acrotreta nicholsoni Davidson, Swanston, 1877, Proc. Belfast Natm-alists' Field Club, Appendix 4 for 1876-77, PI. VII, 



figs. 21a-c. (Not described. Figs. 21b and 21c are copied from Davidson, 1868, PI. XVI, figs. 14a and 14b.) 

 Acrotreta nicholsoni Davidson, 1883, British Fossil Brachiopoda, vol. 5, pt. 2, p. 213, PI. XVI, figs. 21-23. (New 



localities mentioned and new figm-es given. Figs. 21a, 21c, 21b, 22a, 22b, 21d, and 22c are reproduced in this 



monograph, PL LXXIII, figs, le-lk, respectively.) 

 Acrotreta nicholsoni Davidson, Swanston, 1886, Systematic Lists, Flora, Fauna, Paleontology, and Archaeology, North 



of Ireland, vol. 1, PL VII, figs. 21a-c. (Reprint of Swanston, 1877, PL VII, figs. 21a-c.) 

 Acrotreta nicholsoni Davidson, Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 103, fig. 59. 



(Mentioned in the text and fig. 59 copied from Davidson, 1868, PL XVI, fig. 16a.) 



The original description by Davidson follows : 

 Shell small, about 2 lines in length by about the same in breadth, almost circular; rather wider and broadly rounded 

 anteriorly; nearly straight posteriorly; dorsal valve very slightly convex; ventral valve conical; apex subcentral 

 and truncated by a minute circular foramen, situated at a little more than one- third of the length of the valve. Prom 

 the center of the posterior margin a narrow groove or channel extends to the base of the foramen, while on either side 

 a small, flattened triangular space or false area (?) is limited by an indented line. Sm-face of both valves marked 

 with numerous concentric lines of growth. 



Subsequently Davidson obtained some less-compressed specimens and reillustrated [1883, 

 PI. XVI, figs. 21-23] the species. In order to bring before the student all available information 

 I have reproduced most of Davidson's figures. 



Olservations. — A. nicholsoni is one of the largest species of the genus and one of the latest 

 in point of time. Davidson [1883, PI. XVI, figs. 21-23] illustrates specimens 5 to 6 mm. in 

 length, and it ranges up into the Llandeilo. 



The strongly marked false area and the high ventral valve serve to distinguish this species. 



The specific name was given in honor of Nicholson. 

 Formation and locality. — Lower Ordovician: (316 [Daiidso7i, 1871, p. 34S]) Upper Llandeilo (?) blacl shales, at 

 Dobbs Linn, near Moffat, Dumfriesshire; and (316a [Davidson, 1883, p. 213]) Llandeilo at Craighead, Ardmillan Brae, 

 and Balcletchie, all in the Girvan district of Ayrshire; both in Scotland. 

 (317 [Davidson, 1883, p. 213]) Coalpit Bay, County Down, Ireland. 



Tipper Cambrian: (304h) Shineton shales at Mary Dingle, South Shi'opshii-e; and (304a [Groom, 1902, p. 110]) 

 "BronsU" shales, Malvern Hills; both in England. 



Acrotreta nox Walcott. 

 ^ Plate LXXVII, figure 10. 



Acrotreta nox Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 301. (Characterized as below as a new species.) 



This minute shell, 1 mm. in length, is characterized by its low ventral valve, nipple-like 

 apex, and apparently smooth surface. 



Formation and locality. — Upper Cambrian : (lOv) Shales in " St. Croix sandstone, ' ' at Fox Glen, 8 miles (12.8 km.) 

 east of Baraboo, Sauk County, Wisconsin. 



