ACEOTRETID^. 715 



Wiman states [1902, p. 54] that this form is quite Uke Acrotreta uplandica, but lower, as 

 the height is only one-third of the diameter. It is so close in all other respects that I do not 

 think that more than a varietal value should be given to the differences mentioned. 



The specific name is derived from Limon Island, Sweden. 



Formation and locality. — Middle? Cambrian: (311n) Drift bowlder of bluish calcareous sandstone, No. 6 

 [Wiman, 1902, p. 57], on Limon Island, about 12 miles (19.3 km.) east-northeast of Gefle, Province of Gefleborg, Sweden. 



:. ACEOTEETA Sp. THldt. 



Acrotreta cf. socialis Matthew [not Linnarsson or von Seebach], 1902, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, 



pt. 5, No. 20, pp. 392-394, PI. XV, figs. 5a-k. (Described and discussed.) 

 Acrotreta cf. socialis Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 183-185, PL III, 



figs. 5a-k. (Copy of preceding reference.) 



Matthew describes and illustrates this form and compares it with Acrotreta socialis von 

 Seebach. A comparison of his specimens with A. socialis shows the two forms to be distinct. 

 Matthew's specimens are more suggestive of A. sagittalis magna (Matthew) (PI. LXVI, figs. 

 4, 4a-f ) except that the ventral valve is higher. 



With the original specimens for study it appears best to indicate this form as an unde- 

 termined species. 



Formation and locality. — Middle Cambrian : (336a [llatthew, 1903, p. 184]) Shales of Division C2c of Matthew's 

 Bretonian, on the eastern slope of the valley of McNeil Brook, on the road to Trout Brook, Cape Breton, Nova Scotia. 



AcEOTEETA sp. undt. Westergard. 

 y 



Acrotreta sp. WbstergIrd, 1909, Medd. fran Lunds Geol. Faltklubb, Ser. B, No. 4 (Aftryck ur K. Fysiografiska 

 Sallskapets HandL, N. F., Bd. 20), pp. 57 and 77, PL II, figs. 25a-b. (Locality mentioned in Swedish.) 



This species is not described by Westergard, and I have not seen the specimen. It is of 

 medium height and appears to be a true Acrotreta. 



Formation and locality. — TTpper Cambrian: (310t) Sabzone c of the Dictyograptus slate at Fogelsang; (310u) 

 subzone c of the Dictyograptus slate at Jerrestad; and (310v) subzone 6 of the Dictyograptus slate at Tosterup; all 

 Westergard, 1909, pp. 57 and 77] in the Province of Malmohus, Sweden. - 



Genus ACROTHYBA Matthew.o 



[Sjipog, at the top: and dupa, a door.] 



Acrothyra Matthew, 1901, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 4, No. 19, pp. 303-304. (Described and 

 discussed as a new genus.) 



Acrothyra Matthew, 1902, Trans. Roy. Soc. Canada for 1902, 2d ser., vol. 8, sec. 4, No. 3, p. 104. (Notes on variation 

 in size, etc., of the different species and varieties of Acrothyra.) 



Acrothyra Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 85-86. (Copied from Mat- 

 thew, 1901b, pp. 303-304.) 



Acrothyra Matthew, 1903, idem, pp. 88-89. (Copied from Matthew, 1901b, p. 304.) 



Acrothyra Matthew, Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 146. (Classifica- 

 tion of genus.) 



Ventral valve elongate conical, with the posterior face more or less flattened to form a 

 false area marked on the median fine by a shallow groove. Area unknoAvn. The slope of 

 the false area is usually backward so as to throw the apex back of the posterior margin, but 

 in some instances it is directly on a line with the margin. Pedicle opening minute, shghtly 

 truncating the apex. Dorsal valve strongly convex with the beak marginal. Area unknown. 



Surface marked by fine, concentric strise and fines of growth and very fine, irregular, con- 

 centric strife that inosculate so as to give a granulose or pitted surface when seen by a strong 

 lens. The inner surface of the ventral valve shows fine radiating lines in some specimens. 

 The shell is buUt up of several thin layers or lamellae of a calcareocorneous character. The 



o The synonymy for this genus does not give a complete record of the various genera under which the species now included in Acrothyra were 

 fonneriy placed; it gives only those references in which the genus is discussed or described. To complete the record the following mere generic 

 references are listed: Acrotreta Matthew [1899b, p. 203; 1902b, pp. 390 and 391; 1903, pp. 73 and 95]. 



