ACROTRETID^. 637 



the upward arching of the margin of the ventral valve. Surface of shell beautifully ornamented 

 by elevated sharp oblique lines with deep interspaces that give a strongly punctate appear- 

 ance to the shell; concentric undulations and ridges of growth also occur in a more or less 

 irregular manner. The inner layers or lamellae are marked by fine, concentric, and rather 

 strong radiating lines. The shell is built up of numerous thin layers or lamellae of a corneous 

 appearance. 



A slightly compressed ventral valve has a length of 5 mm. with a width of 6.5 mm, A 

 ventral valve is 4 mm. in length with a width of 5 mm. 



Observations. — This is one of the most beautifully ornamented species of the genus. Its 

 surface is not unlike that of some varieties of Micromitra (LpJiidella) pannula (White). In 

 form it suggests Acrothele (RedlicJieTla) granulata (Linnarsson). Nothing is known of the 

 interior character of the valves. 



Formation and locality. — lower Cambrian : (8v) Shales in upper portion of Holmia kjerulfi zone, Ringsaker, 

 Province of Hedemarken, Norway. 



Acrothele belltila Walcott. 

 Plate LVIII, figures 5, 5a-h. 



Acrothele bellula Walcott, 1897, Proc. U. S. Nat. Mus., vol. 19, pp. 716-717, PI. LX, figs. 4, 4a-e. (Described and 

 discussed as a new species. Figa. 4, 4a-e are reproduced (with slight changes) in this monograph, PL LVIII, 

 figs. 5, 5a, 5c, 5f-h, respectively.) 



General form transversely broad oval, with the posterior margin slightly transverse. 

 Ventral valve slightlj'' conical with the apex between the posterior seventh and eighth of the 

 length of the valve; the slope from the apex to the posterior margin is about 45° and to the 

 front at a low angle after passing from the steeper slope adjoining the apex. On each side 

 of the apex there is an elongate, sharp tubercle, and just back, a minute pedicle opening. A 

 shghtly outlined triangular false area begins at the pedicle opening and extends to the posterior 

 margin; it is divided by a faint, narrow, rounded ridge. Dorsal valve moderately convex, with 

 the umbo broad, and beak at the posterior margin. 



The exterior surface is marked by concentric lines of growth that cross the area of the 

 ventral valve. The ventral valve has numerous low, narrow, obscm-e radiating ridges, and 

 an extremely minute series of irregidar, concentric, inosculating elevated lines that give the 

 surface a roughened appearance under a strong lens. 



Substance of the shell in appearance corneous. The largest shell in the collection is repre- 

 sented by a dorsal valve; this has a length of 6 mm.; width, 7 mm. 



The casts of the interior of the ventral valve are very good; they show that the sheU was 

 thick over the umbo, that the cast of the pedicle opening was cone-shaped (PI. LVIII, fig. 

 5b), and that on each side just in front of the base of the opening there was a short, narrow 

 depression corresponding hi position to the tubercles on the outer surface. Back of the cast of 

 the opening the false area is rather strongly defined, also the cast of the hollows occupied by the 

 cardinal muscles; the main vascular sinuses originate just back of the pedicle opening and arch 

 forward toward the anterolateral margins of the shell (PL LVIII, fig. 5e) ; usually they have 

 left no impression on the shell in advance of the visceral area; the visceral area is short and 

 fairly well defined on some specimens (fig. 5d). Nothing is known of the muscle scars of the 

 ventral valve except the position of the cardinal scars. 



The cast of the interior of the dorsal valve is most interesting, as a true area similar to that 

 of Acrotreta definita Walcott (PI. LXIV, fig. 2d) and Acrotreta liutorgai Walcott (PI. LXV, fig. 3j) 

 is clearly shown; the area is short and divided midway by a triangular space, the lateral, 

 slightly elevated sides of winch project a little into the valve beyond the front margin, very 

 much as in Oholella ailantica Walcott (PI. LV, fig. le) and 0. crassa (Hall) (PI. LIV, fig. 2g). 

 A median ridge extends from in front of the area to the anterior third of the valve; it has a 

 narrow, low, median septum on its posterior half, and two central muscle scars (h) on its outer 

 slopes a httle back of the center of the valve; the anterior lateral muscle scars (j) are a little in 



