700 CAMBRIAN BRACHIOPODA. 



Shell minute, subcircular. Ventral valve relatively depressed, the highest point being at 

 the umbo, from which there is a slight downward curvature to the apex. False area low and 

 about vertical, the apex being on a line with the posterior margin. Dorsal valve slightly convex; 

 beak marginal. Surface marked by fine concentric striae. 



Observations. — This minute species occurs with Agnostus Isevigatus in the upper portion of 

 the Middle Cambrian. It is of the Acrotreta sagittalis (Salter) type, but does not appear to be 

 identical with the young of that species. 



Formation and locality. — Middle-Upper Cambrian: (310i) Passage beds between the Middle Cambrian Para- 

 doxides forchhammen zone and the Upper Cambrian Olenus truncatus zone, limestone at Borgholm, Oeland Island, 

 Sweden. 



Middle Cambrian: (320) Gudhem, 12.5 miles (20.1 km.) south-southeast of Skara; and (320a) Djupadal, 19 miles 

 {S0.6 km.) south-southeast of Skara; both [Wallerius, 1895, p. 66] in the Province of Skaraborg, Sweden. 



' Acrotreta prim^va Walcott. 



Plate LXIX, figures 1, la-f. 



Acrotreta gemma Walcott (in part) [not Billings], 1886, Bull. U. S. Geol. Survey No. 30, pp. 98-99, PI. VIII, figs. 

 1, la-b. (Specimens now referred to Acrotreta pyxidicula and A. attenuata were included with the specimens 

 representing J., primseva when this description of A. gemma was written, but they were not figured. The two 

 specimens represented by figs, lb and 1, la are redrawn in this monograph, PL LXIX, figs. Ic and 1, respectively.) 



Acrotreta gemma Walcott (in part) [not Billings], 1891, Tenth Ann, Kept. U. S. Geol. Survey, p. 608, PL LXVII, 

 figs. 5c, 5d, and 5e (not figs. 5, 5a-b, see below). (Figs. 5c, 5d, and 5e are copied from figs, la, 1, and lb, respec- 

 tively, of preceding reference. Specimens now referred to Acrotreta attenuata, A. idahoensis alta, and A. curvata 

 were included with the specimens representing A. primseva in this reference to A. gemma, but only the last two 

 were figured: A. idahoensis alta, PL LXVII, figs. 5 and 5a, and A. curvata, PL LXVII, fig. 5b.) 



Acrotreta gemma Walcott (in part) [not Billings], 1899, Mon. U. S. Geol. Survey, vol. 32, pt. 2, p. 449, PL LXII, 

 figs. 2, 2b, and 2d (not figs. 2a, 2c, and 2e, see below). (Figs. 2, 2b, and 2d are copied from Walcott, 1886b, PL 

 VIII, figs. 1, la, and lb, respectively. Specimens now referred to Acrotreta pyxidicula, A. attenuata, A. idahoensis 

 alta, and A. curvata were included with the specimens representing A. primseva when this description of A. 

 gemma was written, but only the last two were figured; A. idahoensis alta, PL LXII, figs. 2a and 2c, and A. 

 curvata, PL LXII, fig. 2e.) 



Acrotreta primaxa Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 593. (Described essentially as below as a new 

 species.) 



Acrotreta gemma Walcott, Grabau and Shimer [not Billings], 1907, North American Index Fossils, voL 1, p. 199, 

 figs. 234a-c. (Described, and figures copied from Walcott [1886b, PL VIII] figs, la, 1, and lb, respectively.) 



Outline of valves transversely oval to subcircular. Ventral valve subcorneal, with the 

 apex at the summit of the false area near or just above the posterior margin. The elevation 

 varies from one-half to two-thirds the diameter of the shell. The false area is quite clearly 

 defined in most specimens; it varies in width at the posterior margin from one-third to nearly 

 one-half of the diameter of the shell; the path of advance of the pseudodeltidium is marked 

 usually by a narrow incised Ime similar to that of Acrotreta attenuata Meek. Foraminal aper- 

 ture minute and situated at the apex of the cone. Dorsal valve slightly convex, with a minute 

 beak slightly incurved over the posterior margin. A broad, slightly defined median sinus 

 flattens the front and central portion of the valve, but it is not perceptible toward the \imbo 

 and beak. Surface of the shell marked hj fine concentric strias and occasional ridges of growth ; 

 on some shells there is a very fine, almost microscopic, irregular, concentric striation that gives a 

 fretted surface somewhat similar to that of Oholus (Westonia) ella (Hall and Whitfield). The 

 inner lamellae of the shell and the interior surface have traces of fine radiating striae that, with 

 the irregular concentric stria?, give a broken, subimbricated, fretted effect to the surface. The 

 shell is formed of a thin outer layer and several thin inner layers or lamellae, the outer ones 

 of which are slightly oblique to the outer surface layer. The largest shells have a transverse 

 diameter of 5.5 mm., longitudinal diameter 5 mm., height of ventral valve 3 mm.; dorsal valve 

 1 mm. The interior of a slightly crushed ventral valve, figure le, shows the cast of an apical 

 callosity, strong vascular canals, and faint outlines of the visceral cavity. The cast of an 

 interior of a dorsal valve has a long, well-defined median ridge, cardinal scars, and faintly 

 defined central scars, as illustrated by figures Id and If. Interiors of the ventral valve from near 



