678 CAMBRIAN BRACHIOPODA. 



medium-sized, elevated cardinal muscle scars. An interior cast of the dorsal valve indicates 

 a median ridge that extended about four-fifths of the distance from the area to the front mar- 

 gin. It also indicates medium-sized, elevated cardinal muscle scars. A second specimen 

 shows a shoi-ter median ridge, with small, elevated central muscle scars about halfway between 

 the posterior and front margins of the shell. 



Observations . — This species belongs to the Acrotreta sagittalis (Salter) group. The low 

 convexity of the ventral valve, posterior position of the apex, the strong cardinal muscle 

 scars, and the strong median ridge, and the cardinal and central scars of the dorsal valve, are 

 all features common to Acrotreta hellatula and A. sagittalis. It differs from A. sagittalis in 

 the more posterior position of the apex and less convexity of the ventral valve, and from 

 A. definita Walcott in its much smaller size and less elevated ventral valve. 



Formation and locality. — Middle Cambrian: (lOz) About 2,900 feet (884 m.) above the Lower Cambrian and 

 1,500 feet [457.2 m.) below the Upper Cavibrian in la of the Marjum limestone [Walcott, 1908f, p. 179]; (lln) a horizon 

 about 100 feet (30.5 m.) above (lOz); and (3w) about 2,350 feet (716.3 m.) above the Lower Cambrian and 2,050 feet 

 (624.8 m.) below the Upper Cambrian in the central portion of Ic of the Marjum limestone [Walcott, 1908f, p. 180], 

 in the long cliff about 2 miles (3.2 km.) southeast of Marjum Pass; both in the House Range [Walcott, 1908f, Pis. XIII 

 and XV], Millard County, Utah. 



Acrotreta belti (Davidson). 



I , '^'' Plate LXXVII, figures 8, 8a. 



Obolella belti Davidson, 1868, Geol. Mag., vol. 5, pp. 310-311, PI. XV, figs. 25-27. (Described as below as a new 

 species. The specimens represented by figs. 25-27 are redrawn by Davidson, 1871, PL L, figs. 15, 17, and 16, 

 respectively, and the last two figures are copied in this monograph ; see note accompanying the following reference . ) 



Obolella belti Davidson, 1871, British Fossil Brachiopoda, vol. 3, pt. 7, No. 4, pp. 340-341, PL L, figs. 15-17. (Copy 

 of preceding reference; figs. 15-17 are copied (with slight changes) from figs. 25, 27, and 26, respectively, of 

 the preceding reference. Figs. 16a and 17a are reproduced in this monograph, PL LXXVII, figs. 8 and 8a, 

 respectively.) 



Obolella sagittalis belti Davidson, 1871, idem, description of figures, opposite PL L. (Species placed as a variety 

 without explanation.) 



Not Linnarssonia beltif Matthew, 1892, Trans. Roy. Soc. Canada for 1891, 1st ser., vol. 9, sec. 4, No. 5, pp. 42-43, 

 PL XII, figs. 7a-c. (This species is not specifically referred in this monograph.) 



Linnarssonia 6eZ<i (Davidson), Matley, 1902, Quart. Jour. Geol. Soc. London, vol. 58, pt. 1, p. 145, figs. 17-18. (Char- 

 acterized.) 



Not Linnarssonia of. belti Matthew, 1903, Geol. Survey Canada, Rept. Cambrian Rocks Cape Breton, pp. 209-210, 

 PL XVI, figs. 3a-c. (Copies Matthew, 1892, pp. 42-43, PL XII, figs. 7a-c. The species is not specifically 

 referred in this monograph.) 



The original description by Davidson follows: 



Shell small, less than a line in length by about 1 line in breadth; transversely oval; beak acuminated; front 

 broadly rounded. Valves moderately convex and marked by concentric lines of growth. The internal characters 

 agree pretty closely with those described in 0. sagittalis. 



Observations. — It is possible that with a good series of specimens of this species its relations 

 to other species might be more accurately determined than Doctor Davidson was able to do. 

 It is not probable that .^croireta sagriitoKs (Salter) persisted into Lower Tremadoc time, but with 

 a genus where the specific characters are so limited and the variations within the same species 

 considerable it is exceedingly difficult to determine the specific relations of a form represented 

 by only a few not very good specimens. Acrotreta belti appears to belong to the A. sagittalis 

 group of forms, but for the present I think it is quite as well to leave it as a species. 



Formation and locality. — Upper Cambrian: (305 [Davidson, 1871, -p. S41-]) Lower Tremadoc shales, Craig-y-dinas, 

 North Wales. 



(304 [Matley, 1902, p. 145]) "Bronsil" shales at White Leaved Oak, Malvern 'Hills; and (304a [Groom, 1902, 

 p. 110]) "Bronsil" shales in the Malvern Hills; both between Herefordshire and Worcestershire, England. 



Acrotreta bisecta Matthew. 

 Plate LXVI, figures 7, 7a-e. 



Acrotreta baileyi ? Matthew, 1892, Trans. Roy. Soc. Canada for 1891, 1st ser., vol. 9, sec. 4, No. 5, p. 43, PL XII, fig. 7d. 



(Characterized and discussed.) 

 Acrotreta bisecta Matthew, 1901, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 4, No. 19, pp. 27^-276, PL V, figs. 



5a-g. (Described and discussed as a new species.) 



