ACROTRETIDiE. 697 



AcROTRETA CELANDiCA Westergard. 



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

 Sallskapets Handl., N. F., Bd. 20), p. 76, PI. V, figs. 24a-b. (Described in Swedish as a new species; see below 

 for translation.) 



The original description hj Westergard follows: 



Apex situated near the posterior boi-der. Along the latter there is a faintly defined triangular area which occupies 

 one-fom-th of the periphery of the shell. The shell is about half as high as broad. Surface smooth, with fine distinct 

 growth lines, which, although more faintly, are also to be seen on the area. The slope from the apex to the anterior 

 border is slightly concave, though this is likely a secondary phenomenon due to pressure. The apex itself has not 

 been preserved. 



Formation and locality. — ^Tipper Cambrian: (310r) Zone v of the Ceratopyge slate at Ottenby, Oeland Island; and 

 (310s) uppermost part of the alum slate in beds equivalent to the Ceratopyge slate, northern part of Oeland Island; 

 both [Westergard, 1909, p. 76] in Sweden. 



Acrotreta ophirensis Walcott. 



Plate LXXIV, figiures 1, la-p. 

 Acrotreta ophirensis Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 591-592. (Described as below as a new species.) 



General outline transversely broad oval, sometimes nearly circular, with the posterior 

 margin slightly indented midway on the ventral valve by incurving to the false pedicle furrow. 

 The ventral valve is convex and moderately elevated, the highest point being in front of the 

 pedicle aperture at about one-third the diameter of the shell. Pedicle aperture large for the 

 size of the shell and opening either directly or obliquely backward; one specimen shows an 

 obscure, short, narrow, triangular false area, with a vertical furrow crossing it (PL LXXIV, 

 fig. la"); false area scarcely defined by the cardinal slopes, which incurve very gently ; median 

 furrow well defined, rather strong, and nearly flat on the bottom, the margins being sharply 

 outlined in many specimens. Some of the shells curve over the false area so that the depressed 

 apex extends slightly beyond the posterior margin, but generally it is directly on the line of, 

 or a little in front of, the posterior margin. 



Longitudinal diameter of average size ventral valve 3 mm., with a length of 2 to 2.5 mm.; 

 elevation, 1.5 mm. A few shells have nearly the same length and width. The convexity of 

 the dorsal valve averages 0.75 mm. The minute beak of the dorsal valve curves down to the 

 posterior margin from the somewhat swollen posterior third of the valve. 



Surface marked by fine concentric stride and lines, of growth, some of which form con- 

 centric ridges. The shell is built up of a thin outer layer and numerous thin inner layers or 

 lamellae that are oblique to the outer layer over the central and outer portions, the obhquity 

 increasing toward the outer anterior and lateral margins. 



The interior of the ventral valve shows a rather strong apical callosity that extended 

 nearly to the posterior inner margin of the shell; distinct, but relative^ small cardinal scars, 

 and narrow main vascular sinuses that maj' be traced nearly to the anterolateral margins of 

 the valve. The outlines of the visceral cavity are indicated on one well-preserved cast of the 

 interior of the valve (PI. LXXIV, fig. le). The interior of the dorsal valve shows great vai'ia- 

 tion in the size and length of the median ridge and cardinal and central scars; these characters 

 are fully shown in the numerous illustrations of the interior and casts of the interior of the 

 dorsal valve. 



Observations. — This species is most closely related to Acrotreta curvata Walcott. It diff'ers 

 in the ventral valve being larger, in the apex being less extended over the false area, and the 

 form of the median furrow, which is like that of A. attenuata Meek. Although the shells occur 

 in a compact unaltered limestone, there is more or less distortion in the outline of both valves; 

 this, taken in connection with the variation in form and size of the vascular markings, muscle 

 scars, and median ridge of the dorsal valve, might serve to discriminate several so-called varie- 

 ties, but I have given names to only two of the more pronounced varieties. 



