644 CAMBEIAN BRACHIOPODA. 



ACROTHELE DECIPIENS Waicott. 

 ''' Plate LVIII, figures 3, 3a-d. 



Acrothele decipiens Walcott, 1897, Proc. U. S. Nat. Mus., vol. 19, p. 716, PI. LX, fig. 2. (Described and discussed 

 as a new species. The specimen represented by fig. 2 is redrawn in this monograph, PL LVIII, fig. 3a.) 



General form of the ventral valve subcircular, usually a little longer than wide; moderately- 

 convex; apex at the posterior one-sixth to one-eighth of the valve. Pedicle aperture of medium 

 size, situated on the posterior slope of the summit of the apex; the slope fi'om the apex to 

 the posterior margin is rather abrupt; and very gentle to the front margin. The obscure 

 false area, with a faintly defined groove down its center, is shown on some specimens. Dorsal 

 valve oval, moderately convex, and marked by a shallow depression extending from the back 

 to the front; apex marginal. 



Surface marked by fine, concentric Hnes of growth, and very fine, somewhat irregular, 

 and sometimes inosculating ridges upon which occur exceedingly minute tubercles which give 

 a very finely tuberculated surface; some specimens show depressed, narrow, rounded ridges 

 extending from near the apex to the margins; these ridges vary in size and number. Shell 

 substance corneous. 



Nothing is known of the interior of the ventral valve except a cast of the apical callosity 

 (PI. LVIII, fig. 3a). The interior of the dorsal valve has a strong median ridge that extends 

 forward beyond the center, and strong cardinal scars. 



Observations. — This species is of the Acrothele matthewi (Hartt) type. It has a similar 

 surface, and the apex of the ventral valve is well toward the posterior margin. The interior 

 of the dorsal valve suggests that of Acrotreta more than do most interiors of the dorsal valves 

 of Acrothele. It has an unusually long and strong median ridge and cardinal muscle scars. 



Acrothele decipiens was found associated with fragments of Olenellus in the fine gray hme- 

 stone in the Lower Cambrian shale. 



Formation and locality. — Lower Cambrian: (50) Shaly limestones, 1.125 miles (1.8 1cm.) north of S toner, 10 

 miles {16.1 km.) east-northeast of York; and (346) limestone and shale a little south of Emigsville; both in York County, 

 Pennsylvania. 



Acrothele dichotoma Walcott. 

 L/ 



Plate LX, figure 2. 



Acrothele f dichotoma Walcott, 1884, Mon. U. S. Geol. Survey, vol. 8, pp. 14-15, PI. IX, fig. 11. (Described and 

 discussed as a new species. The specimen represented by fig. 11 is redrawn in this monograph, PI. LX, fig. 2.) 

 Acrotreta dichotoma Walcott, 1886, Bull. U. S. Geol. Siu-vey No. 30, p. 107. (Suggests reference to Acrotreta.) 

 Acrotreta dichotoma (Walcott), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 100 and 

 103. (Mentioned.) 



Shell of medium size, thin, corneous, nearly circular in outUne, the transverse diameter 

 beinc a little greater than the longitudinal. Ventral valve depressed, discoid, with the apex 

 but slightly elevated above the general plane of the surface and situated about one-third the 

 distance from the posterior to the anterior margin. The apex is exfoliated, but the inner layers 

 show that it was perforate and that in the interior the shell was thickened about the pedicle 

 opening. The slightly depressed area is triangular in form, and extends to the posterior 

 maro-in, gradually expanding to a width of a little less than its length; it appears to have 

 been quite minute at first, just beneath the apex, and to have been filled in by successive addi- 

 tions of the shell, so that the generally circular outhne of the shell has scarcely been broken 

 by the very slight truncation of the posterior margin. The additions to the posterior margin 

 of the false area are crowded together as little transverse ridges with fine strise between. 



The outer portion of the shell is largely exfoliated, but traces of radiating strise and con- 

 centric lines of growth are shown. The latter on the outer portion appear to have been con- 

 tinuous with those crossing the pseudodeltidium. From each side of the apex a narrow, main 

 vascular sinus gently diverges, extending toward the central portion of the shell. Transverse 

 diameter, 7.5 mm.; longitudinal diameter, 6.75 mm.; elevation or convexity, about 0.75 mm. 



