part 4] CAMBRIAN HORIZONS OF COMLET. 347 



situated over or behind the posterior margin of the opening of the 

 valves, never in front of it, and that the anterior slope is distinctly 

 convex. In all other respects the shells accord with Walcott's 

 figures and description. 



Dimensions. — The lengths observed for ventral valves vary 

 from 1 to 3 mm. ; the width is 1'25 of the length, and the height 

 is about '5 of the length. 



Locality and horizon. — Comley, Middle Cambrian; from the 

 JBillingsella Beds, horizon Be, also doubtfully from the Parado- 

 xides-davidis Flags, horizon Blx. 



Acrotreta socialis Seebach (?). (PL XXII, fig. 35.) 



Acrotreta socialis Seebach, 1865, Zeitschr. Deutsch. Geol. Geseliscli. vol. xvii, 



p. 341 & pi. viii a, tigs. 1-1. 

 Acrotreta socialis Seebach, Walcott, 1912, U.S. Geol. Surv. Monogr. vol. li, 



p. 711 & pi. lxxii, figs. 3, 3a-3c, 4, 4a-4e. 



Some small ventral valves, found associated with Acrothyra 

 comleyensis and Dorypyr/e la/cei, agree with some of the figures 

 of this species given by Dr. Walcott. 1 The anterior and posterior 

 slopes are nearly straight, the apex is well forward from the posterior 

 margin; the foramen is distinctly visible behind it, the height is 

 about a half of the length, the surface is striated with fine concentric 

 lines, the false area is moderately Avell-defined, and the vertical 

 groove is narrow. 



Dimensions. — Length=2 mm., width=2 mm., height="8 mm. 



These shells differ from Walcott's description of Seebach's 

 species in being rather smaller, in having finer concentric striae, and 

 in the narrower vertical groove. 



From A. schmalenseei they differ very decidedly in the position 

 of the ajiex and in the anterior slope being almost straight, instead 

 of curved. 



Locality and horizon. — Comley, Middle Cambrian; from 

 the Dorypyge-laJcei Flags, Ba.. 



Acrotreta sp. indet. a. (PL XXII, fig. 33.) 



A few valves have been found in association with A. scJima- 

 lenseei var. matleyi, which appear to be distinct from any described 

 species, but are too scarce or indifferently preserved to be definitely 

 named. 



They are distinguished by (1) the ventral valve being very 

 elevated (it is almost as high as it is long) ; (2) the straight anterior- 

 slopes ; (3) the false area being inclined backwards ; (4) the 

 absence of the vertical groove on the false area in the only exterior 

 known. 



The exterior is marked by fine concentric lines, set more widely 

 apart than in Acrotreta schmalenseei. 



Dimensions. — Length = Vo mm.; width = 1/5 mm.; height 

 = 1/25 mm. 



1 1912, pi. lxxiii, figs. 4, 4', 4', 4 a, 4 a', 4 a". 



