ACRQTRETIDiE. 631 



Acrothele Lmnarsson, Davidson, 1883, British Fossil Brachiopoda, vol. 5, pt. 2, pp. 213-214. (Copies description 

 given by Linnarsson, 1876, p. 20.) 



Acrothele Linnarsson, Walcott, 1886, Bull. IT. S. Geol. Survey No. 30, pp. 107-108. (Copies original description 

 [Linnarsson, 1876, p. 20] and discusses genus.) 



Acrothele Linnarsson, Obhleet, 1887, Manuel de conchyliologie, by Fischer, pp. 1269-1270. (Described in French.) 



Acrothele Linnarsson, Hall and Clarke, 1892, Eleventh Ann. Kept. State Geologist New York for 1891, pp. 249-250. 

 (Described.) 



Acrothele Linnarsson, Hall and Clarke, 1892, Forty-fifth Ann. Kept. New York State Museum for 1891, pp. 565-566. 

 (Copy of preceding reference.) 



Acrothele Linnarsson, Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 98-101. (Copies 

 original description, Linnarsson, 1876, p. 20, and discusses genus.) 



Acrothele Linnarsson, Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 103-105. (Dis- 

 tribution discussed and tabulated.) 



Acrothele Linnarsson, Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 200. (Described.) 



Linnarssonia Grabau and Shimer?, 1907, idem, p. 200. (Described, but the only species referred to the genus {Lin- 

 narssonia pretiosa) is not figured, and it is impossible to tell whether the authors are discussing the L. pretiosa 

 that is now referred to the genus Acrotreta (Acrotreta sagittalis), or to the genus Acrothele (Acrothele pretiosa).) 



Acrothele Linnarsson, Walcott, 1908, Smithsonian Misc. CoU., vol. 53, No. 4, PI. XI, and pp. 142 and 146. (Classi- 

 fication of genus.) 



General form subcircular to transversely broad oval in outline. Ventral valve moderately 

 convex to subconical; a false area is usually more or less distinctly outlined on the ventral 

 "valve between the apex (beak) and the posterior margin; pedicle opening on the posterior 

 slope of the apex (or beneath the beak). The only suggestion of a true cardinal area is in 

 Acrothele hellula Walcott (PI. LVIII, fig. 5c); and this appears to be a broadening and slight 

 flattening of the posterior under edge of the shell. Dorsal valve gently convex, and some- 

 times nearly flat; beak minute, marginal; area known only in A. hellula Walcott (PL LVIII, 

 figs. 5f-h), where it is short and divided midway by a smaU, triangular false deltidium, very 

 much as in Acrotreta definita Walcott (PI. LXIV, fig. 2d.) 



Surface marked by concentric lines and stride of growth which cross the false area of the 

 ventral valve; on some species there are low, rounded, radiating ridges that are usually con- 

 fined to the ventral valve; in addition, on some species, there is a series of more or less inoscu- 

 lating, irregular, fine ridges with tubercles on them, which gives a highly ornamented surface 

 such as occurs on some forms of Micromitra and Westonia. 



The shells of Acrothele rarely exceed 13 mm. in diameter, and the average size of adult 

 shells is from 6 to 8 mm. Substance of shell corneous. The shell is built up of several layers 

 or lameUse that are usually more or less obliquely inchned to the outer surface, over the central 

 and outer portions. 



The interior of the ventral valve has a small visceral area about the pedicle opening and 

 extending a short distance in front of it; in several species an elongate depression occurs on 

 each side and a little in advance of the pedicle opening that corresponds to the tubercle on 

 each side of the apex of the outer surface; frequently the shell is thickened beneath the visceral 

 cavity and a short ridge of varying width and length results; the edge of the pedicle opening 

 may be slightly thickened, which forms a callosity or apical swelling, but not to the same 

 extent as in the ventral valve of Acrotreta. Nothing has been seen of an area in front of the 

 visceral area corresponding to the trapezoidal areas of Oholus, Acrotreta, and Trematobolus, in 

 which the central, outside lateral, and middle lateral muscles were attached ; in fact, it is only very 

 rarely that the impression of the visceral area is shown. The main vascular sinuses start 

 from just back of the pedicle opening, and curve, one on each side, out and then forward into 

 the body of the valve, usually within a line drawn halfway between the center and the outer 

 margin of the valve; many branches from the main sinus occur in some species. The trans- 

 median and anterior lateral muscle scars appear to be merged in the cardinal muscle scar, situ- 

 ated on the outer posterolateral slope, outside of the main vascular sinuses; on one shell what 

 appear to be the umbonal muscle scars are preserved (PI. LXI, fig. Ic). 



The interior of the dorsal valve has a median ridge of varying length and size, and main 

 vascular sinuses that start near the posterior margin and extend directly outward for a short 

 distance, and then obliquely forward in almost a direct line (PI. LVI, figs. Id and le; PI. LX, 

 figs. Ik and 1 1). The cardinal muscle scars are usually small and close to the posterolateral 



