ACROTUETID^. 705 



dorsal valve show a remarkable range of variation in the length and size of the median ridge 

 (PI. LXXI, figs. 3e, 3g-j). Within the extremes of variation specific characters coukl be 

 established. 



This species has had a varied experience at the hands of paleontologists. Da\'idson [1871, 

 PL L, figs. 1-14] elaborately illustrated the casts of the interiors of the valves, and I [1885, 

 p. 116] copied some of his figures when preparing the description of the genus Linnarssonia. 

 When in Wales in 1888, 1 collected a quantity of material at St. Da'snids, and recently Schmalensee 

 collected a good series at Andrarum for the United States National Museum. From these col- 

 lections figures have been drawn that illustrate the appearance of the shells as they occur both 

 in shale and limestone. 



The varieties taconica and transversa are strongly marked, but I think are not entitled to 

 full specific valuation. A direct comparison of specimens from the black shales of Little Metis 

 with those from the black shales of the type locality of A. sagittalis at St. Davids fails to disclose 

 any specific differences between them, so far as the material at hand permits of comparison. 

 The Little Metis shell was published as Linnarssonia cf. pretiosa by Hall [1890, p. 55], and this 

 identification was accepted by Sir William Dawson [1890, p. 53]. "OioleUa pretiosa" Billings 

 [1862d, p. 68] is a true Acroihele, and no species of Acrotreta is known to occur at the type 

 locality on the Chaudiere River. The stratigraphic horizon of the Little Metis beds has not 

 been definitely determined. Dawson [1890, p. 32] says: "At Metis the evidence of the pebbles 

 in the conglomerates indicates that they are newer than the Lower Cambrian, and the few 

 fossils found in the sandstones and shales would tend to place them at or on the horizon of 

 Che Chazy." I personally examined the section at Little Metis in 1899, and am not at all 

 certain of the horizon of the shales carrying Acrotreta sagittalis and the beautiful fossil sponges 

 described by Dawson [1890, pp. 37-55]. It is a region of strong folding and thrust faults. 

 The Acroihele is a Middle Cambrian type, and nothing similar to it is known from the Upper 

 Cambrian. As far as this shell can locate the horizon, it is Cambrian, and probably low down 

 in the Upper Cambrian, if not in the Middle Cambrian. 



Formation and locality. — Passage beds between the Upper Cambrian and the Ordovician: (309 [Moberg and 

 Segerberg, 1906, desc. of PI. I]) Ceratopyge limestone (Zone 4) at Fogelsang, 5 miles (8 km.) east of Lund, Province 

 of Malmohus, Sweden. 



(323 [Wiman, 1902, p. 66]) Phyllograptus slate and Ceratopyge limestone in the Christiania region of Norway; and 

 (323h [Brogger, 1882, p. 17]) blue Ceratopyge limestone at Vestfossen, 10 miles (16.1 km.) west-southwest of Christiania, 

 iN^orway. 



Upper Cambrian: (331e-321m [Wiman, 1902, p. 68]) Drift bowlders of Ceratopyge slate, Nos. 1-4, 6-8, 9, 12, 14, 

 17, .23, and 27, on Biludden, about 20 miles (32.2 km.) east of Gefle, Province of Gefleborg; and (321n [Wiman, 1902, 

 p. 68]) drift bowlder of limestone (Ceratopyge slate horizon), at Trodje, in Gastrikland; both in Sweden. 



(323a [Wiman, 1902, p. 66]) Ceratopyge slate in the Christiania region of Norway. 



(318g [Davidson, 1868, p. 310]) Lower Lingula flags at Porth-y-rhaw, Pen-y-pleidiau, and several other localities near 

 St. Davids, South Wales. 



(318c [Davidson, 1868, p. 310]) Lower Lingula flags at the Rheider Waterfall Valley and at Gwynfynydd and 

 other places near Dolgelly, all in North Wales. 



Middle? Cambrian (Probably between Middle and Upper Cambrian): (392g [Dawson, 1888a, p. 55]) Black shales 

 at Little Metis, Province of Quebec, Canada. 



Middle Cambrian: (2g) Sandstone of Division Ibl of Matthew's [1895a, p. 108] Protolenus zone on Hanford 

 Brook, St. John County; and ,(2s) limestone in upper part of Paradoxides zone, Hastings Cove [Matthew, 1898b, p. 38], 

 on Kennebecasis Bay, 0.5 mile (0.8 km.) northeast of Torrybm^n, on the Intercolonial Railway northeast of St. John, 

 St. John County; both in New Brunswick. 



(6g) Limestone near the base of the Middle Cambrian, the lowest horizon carrying Paradoxides, northwest side 

 of Chappie Arm Harbor, about 1 mile (1.6 km.) from its head, Trinity Bay; (6w) limestone at Seal Point Cove, near 

 Long Point, Trinity Bay; and (61) limestone about 300 feet (91.4 m.) below the base of the Upper Cambrian on shore 

 north of Fosters Point, Random Island, Random Sound; all three in Newfoundland. 



(318h) Shales in the Menevian at St. Davids, South Wales. 



(316b [Davidson, 1883, p. 211]) Cah-n Burn and Druidhill Burn, both in Dumfriesshire, Scotland. 



(8w) Limestones of the Paradoxides forchhammeri zone, at Andrarum, 20 miles (32.2 km.) northwest of Simrishamn, 

 Province of Christianstad ; (321y) shale at Andrarum, 20 miles (32.2 km.) northwest of Simi-ishamn, Christianstad ; 

 and (321z) shales at Fogelsang, 5 miles (8 km.) east of Lund, Province of Malmohus; aU in Sweden. 



82667°— VOL 51, pt 1—12 45 



