ACROTRETIDiE. 



641 



1 



posterior third; the slope from the apex to the posterior margin is about 45° and that to the 

 anterior margin 20°. The posterior slope is marked by a rather clearly defined false area, 

 which is slightly flattened, and marked midway by a very slight depression; the apical opening 

 is small, oval in outline, and situated on the posterior side of the apex. On finely preserved 

 specimens the front and sides of the apical opening have a narrow, slightly elevated ridge, 

 and a trace of a small tubercle on each side at the anterior end of the opening; the posterior 

 margin arches very slightly beneath the false area. The dorsal valve is moderately convex, 

 and most prominent on the umbo, with the apex at the posterior margin. 



One of the largest ventral valves has a length of 9 mm.; width, 10 mm. The dorsal valve 

 has a length of 7 mm.; width, 8 mm. These proportions vary somewhat as the shells are 

 more or less distorted by compression in the shale. Shell substance corneous. 



The exterior surface is marked by very fine, concentric ridges of growth, with depressed 

 and elevated lines between them; there are also numerous low, rounded, narrow, radiating 

 ridges that vary in strength and number on different specimens; on some shells there is hardly 

 a trace of the radiating ridges, and on others they are numerous, especially on the sides; one 

 dorsal valve shows nine on each side, with a few scattered through the central portion in addi- 

 tion to the concentric and radiat- 

 ing lines ; there is also a system of 

 exceedingly minute and irregular, 

 more or less inosculating, but, as 

 a whole, concentric ridges, upon 

 which very minute tubercles oc- 

 cur. These are most perfect over 

 the posterior half of the shell, but 

 extend in many examples to the 

 front margin. 



A cast of fhe apical portion 

 of the dorsal valve shows that the 

 pedicle aperture enlarged as it 

 passed through the shell, and 

 that the visceral area was short. 



Observations. — This species is 

 of the Acrothele coriacea Linnars- 

 son type. It has the same sur- 

 face, except that the irregular, 

 concentric ridges and tubercles 

 are finer. 



The Lower Cambrian specimens from British Columbia appear to be identical with those 

 from Montana and they are associated with the same subfauna, of which ATbertella helena 

 Walcott is considered to be typical. The shells from the Middle Cambrian, wliich occur 2,450 

 feet higher in the section on Mount Stephen, have the same form and surface markings and 

 appear to be identical in all respects. It is interesting to note that Micromitra {Tpliidella) 

 pannula (White) has a similar vertical range. 



The specific name is given for Mr. M. Collen, of White Sulphur Springs, Montana, who 

 collected the finest specimens at the locality on Scapegoat Mountain, Montana. 



Formation and locality. — Middle Cambrian: (14s) About 2,300 feet (701 m.) above tlie Lower Cambrian and 

 2,700 feet (823 m.) below the Upper Cambrian, in the Ogygopsis zone of the Stephen formation [V^'alcott, 1908f, p. 210], 

 on the northwest slope of Mount Stephen, above Field on the Canadian Pacific Railway, British Columbia, Canada. 



(4q) About 315 feet {96 m.) above' the unconformable base of the Cambrian and 190 feet {57.9 m.) above the top of the 

 quartzitic sandstones, in a shale which corresponds in position to the upper part of shale No. 6 {Lower Cambrian) of the 

 Dearborn River section [Walcott, 1908f, p. 202] on the ridge between Cordon and Youngs creeks, about halfway between 

 Gordon Mountain and Cardinal Peak, Ovando quadrangle { U. 8. Geol. Survey), Powell County, Montana. 



62667°— VOL 51, pt 1—12 41 



/ 



C 



Figure 55. — Acrothele colleni n. sp. A, Slab showing a ventral and a dorsal valve (U. S. 

 Nat. Mus. Cat. No. 51410a). B, Broken ventral valve showing false area (U. S. Nat. 

 Mus. Cat. No. 51410b) (X about 4). C, A much larger ventral valve than that represented 

 by B (U. S. Nat. Mus. Cat. No. 514100). D, Cast of a ventral valve shomng the incurv- 

 ing of the flexure lines across the false area (U. S. Nat. Mus. Cat. No. 51410d) (X about 4). 

 E, Dorsal valve (U. S. Nat. Mus. Cat. No. S1410e). 

 The specimens represented are from Locali.ty 35c, a drift block of Lower Cambrian 



shales, on the slopes of Mount Bosworth, on the Continental Divide, 1 mile (1.6 km.) east 



of Hector, British Columbia. 



