ACROTRETIDiE. 679 



Acrotreta hisecta Matthew, 1902, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 5, No. 20, p. 394, PI. XVI, figs. 



2a-g. (Mentioned and figures of preceding reference copied.) 

 Acrotreta dpo Matthew, 1902, idem, pp. 406-407, PL XVIII, figs. 1 and 2. (Described and discussed as a new species.) 

 Acrotreta iisccta Matthew, Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 582. (Described and discussed.) 

 Acrotreta sipo Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 185-186, PI. XVIII, 



figs. 1 and 2. (Copy of Matthew, 1902a, pp. 406-407, PI. XVIII, figs. 1 and 2.) 

 Acrotreta hisecta Matthew, 1903, idem, pp. 186-187, PI. XI, figs. 5a-g. (Copy of Matfrliew, 1901, pp. 275-276, PI. V, 



figs. 5a-g. ) 

 Acrotreta hisecta Matthew, Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 298-299. (Described and discussed as 



below.) 



Nearly all the ventral valves of this species are more or less com]3ressed in the shale, tluis 

 decreasing the true elevation. Matthew [1903, PL XI, lig. 5] illustrates a pointed, high, ventral 

 valve, but does not state whether or not the figure is diagrammatic. Some of the casts in the 

 shale indicate a sharply conical ventral valve. When the apex is broken off the cast of a 

 median apical callosity is seen, with the base of the cast of a medium-sized pedicle tube; the 

 cardinal scars are small and nearty concealed by the cast of the strong main vascular sinuses. 

 There is considerable range of variation in tlie size and length of the median ridge of the dorsal 

 valve. One specimen (fig. 7d) shows a strong median ridge, cardinal and central scars, and 

 deeply excavated false deltidium. Surface marked by fine concentric striae and lines of growth 

 and very fine undulating striae that give the concentric strife a fretted appearance when examined 

 by a strong lens. 



The most nearly related species appears to be A. sabrinse (Callaway) of the Sliineton 

 shales. Matthew [1902a, p. 406] has described a shell as "Acrotreta sipo" that occurs with his 

 " Asaphellus fauna." I am unable to detect any specific differences between it and A. hisecta, 

 from the same area. 



The specific name is due to the presence of a strong median ridge bisecting the dorsal valve. 

 Formation and locality. — Upper Cambrian: (3q) Shale in Barachois Glen, 4 miles (6.4 km.) south of Little 

 Bras d'Or Lake; (101) shale on east bank of Barachois River, 6 miles (9.6 km.) from Little Bras d'Or Lake; (10c and 

 lOd) shales on west side of Barachois River, 0.125 mile (0.2 km.) north of Boisdale Road, opposite McMullin's place; 

 (lOe, lOf, and lOg) shales on the east branch of Barachois River, 0.5 and 0.75 miles (0.8 and 1.2 km.), respectively, 

 north of the crossroad from Boisdale to Upper Leitches Creek; (lOn) shale in ravine on east side of Barachois Glen, 

 3 miles (4.8 km.) from Barachois; (lOh) shale on small east branch of Barachois River, 0.75 mile (1.2 km.) north of the 

 crossroad from Boisdale to Upper Leitches Creek; (13h) shale on east bank of Barachois River, 1.5 miles (2.4 km.) 

 north of Boisdale; (307a [Matthew, 1903, p. 186]) shales of Division C3c2 of Matthew, on McLeod Brook=Barachoi8 

 River; and (307 [Matthew, 1903, p. 7S7]) shales of Division CSc of Matthevi, on McLeod Brooh= Barachois River; all in 

 eastern Cape Breton, Nova Scotia. 



(308 [Matthew, 1892, p. 43]) Shales of Division CSc of Matthew at Navy Island, St. John Harbor, New Brunswick, 

 Canada. 



Acrotreta?? cancellata Walcott. 



Plate LXXIX, figures 5, 5a. 

 Acrotreta ?? cancellata Walcott, 1905, Proc. U. S. Nat. Mus. , vol. 28, p. 299. (Characterized as below as a new species.) 

 Tlie only specimen of this species in the collection has more the form of Micromitra than 

 Acrotreta. In its overlianging false area it suggests Acrothyra. Whatever the genus to which 

 it belongs may be, its cancellated surface serves to distinguish it from forms with wliicli it 

 might otherwise be compared. The surface is formed hj very fine, raised, concentric Hnes or 

 ridges of growth, crossed by sharp radiating lines, wliich are seen only between the concentric 

 hnes. 



Formation and locality. — Passage beds between the Upper Cambrian and the Ordovician: (205) Siliceous 

 limestone on Roundtop Mountain, Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



Acrotreta carinata (Segerberg MS.) Moberg and Segerberg. 



Text figures 56A-D, page 680. 



Acrotreta cf. socialis Broggbr (in part), 1882, Die silurischen Etagen 2 und 3, pp. 46-47, PI. X, figs. 2, 2a-b (not figs. 



3-4, which are not referred in this monograph). (Described in German.) 

 Acrotreta carinata (Segerberg MS.) Moberg and Segerberg, 1906, Medd. fran Lunds Geol. Faltklubb, Ser. B, No. 2 



(Aftryck ur K. Fysiografiska Sallskapets Handl., N. P., Bd. 17), p. 66, PI. Ill, figs. 5a-c and 6. (Described 



in Swedish as a new species; see p. 680 for translation. The figures are reproduced in this monograph as 



figs. 56A-D, p. 680.) 



