ACROTRETID^. 655 



Acrothele proles has the same type of surface as A. avia, also a strong shell that is often 

 thickened in the vicinity of the apex; the number and strength of the radiatiag ridges vary 

 with the thickness of the shell and the manner in which it laas been compressed laterally. 



The position of the apex of the ventral valve varies a little, but on the average it is nearer 

 the posterior margin than in A. avia and not so near as in ^. prima (Matthew). 



Formation and locality. — Middle Cambrian: (13m) o Sandstones of Division ESf in Matthew's [1903, p. 76] 

 Etcheminian, on Gillis Brook, Indian River; and (131') shaly sandstones of Division E3a; and (13n) sandstones of 

 Division ESf; both in Matthew's [1903, p. 21] Etcheminian, on Dugald Brook, Indian River; all in eastern Cape 

 Breton, Nova Scotia. 



ACEOTHELE QUADEILINEATA Pompeckj. 



Plate LVII, figure 5. 



Acrothele quadrilineata Pompeckj, 1896, Jahrb. K.-k. geol. Reichsanstalt for 1895, Bd. 45, Hft. 3, p. 511, PI. XIV, 

 figs. 6a-c. (Described and discussed in German as a new species; see below for liberal translation. Figs. 6b 

 and 6c are reproduced in this monograph, PI. LVII, figs. 5 and 5', respectively.) 



The original description by Pompeckj foUows: 



Ventral valve subcircular in outline; length, 7 mm.; width, 7.8 mm.; posterior margin slightly transverse. 

 Apex low and about 2 mm. from the posterior margin; details of apex unknown; false area narrow and indefinitely 

 defined from the apex to the posterior margin. Four flat ridges radiate from the apex toward the front margin. Sm-face 

 marked by strong, closely arranged, concentric lines that under the microscope show very indistinct granulations. 



Dorsal valve unknown. 



Pompeckj states [1896b, p. 511] that the ventral valve of this species is more arched than 

 that of Acrothele coriacea Linnarsson. It is evidently a forrq, closely allied to the latter and 

 also to Acrothele hohemica (Barrande), which occurs at the same stratigraphic horizon in the 

 " Paradondes green slate." 



Formation and locality. — Middle Cambrian: (345d [Pompeckj, 1896b, p. 511]) Shales of Paradoxides zone, 

 Forester house of Slapy (Buchava quan-y), Skrej, Bohemia. 



Acrothele eaea Walcott. 



Plate LXIII, figures 4, 4a. 



Acrothele rarus Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 303-304. (Described and discussed as below as a 

 new species.) 



Of this shell only the interiors of the valves are known. Shells of average size are about 

 3.5 mm. in diameter. The outline of the valves is subcircular, the transverse diameter being 

 shghtly more than the length of the valve. 



The interior of the ventral valve shows that the valve was moderately convex, with a 

 perforated apex about half a millimeter from the posterior margin. A short, broad median 

 ridge extends for a short distance in front of the foraminal opening, and short, narrow ridges 

 extend obhquely forward from each side of the opening. Wliat may be lateral muscle scars 

 occur close to the posterolateral margins. In the dorsal valve a strong median ridge extends 

 from the posterior margin to the center of the valve; this ridge is angular at the summit and 

 broadest toward its anterior end. A vascular sinus starts on each side of the base of the 

 median ridge and extends obliquely forward. 



The dark interior surface of the valves is marked by concentric lines that give a somewhat 

 laminated appearance to the surface. From the manner in which the shell adheres to the 

 limestone matrix it is probable that its outer surface is roughened by raised lines, somewhat 

 as on Acrothele subsidua (White) . 



The data for comparison of this species with described species from America and Europe 

 are too limited to be of value. A comparison with Acrothele (Redlichella) granulata (Linnarsson), 

 of the Salt Range, India, shows a strong similarity in the interiors of the valves, but I do not 



a 13m is the type locality, though the specimens in the United States National Museum collections to which that number is assigned were 

 collected later than the type specimens. 



