ACROTRETID^. 661 



are identical species; they are associated with quite different faunas and little is known of 

 the specific details of either species. 



Tliis species derives its specific name from its occurrence near Villa Boim. 



Formation and locality. — Lower Cambrian: (351 [Delgado, 1904, p. 365]) Shales at Monte de Valbom, north- 

 east of Villa Boim, Province of Alemtejo, Portugal. 



I ACROTHELE WOODWORTHI Walcott. 



Plate LX, figure 6. 



Acrothele woodworthi Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 88, PL IX, fig. 11. (Described and 

 discussed as below as a new species. Fig. 11 is copied in this monograph, PI. LX, fig. 6.) 



Ventral valve transversely and irregularly oval in outline, convex, with the apex a little 

 back of the center of the valve; pedicle opening, as indicated by the matrix of the exterior, 

 just back of the apex; a subtriangular, gently convex false area is obscurely outlined by lines 

 radiating from the apex to the posterior margin; tlie posterior margin is arched upward one- 

 third or more of the distance from the plane of the margin of the valve to the apex. 



Surface marked by concentric growth lines with fine striae between, and a number of low, 

 rounded, more or less obscure, radiating ribs. The shell is relatively thick and replaced by 

 the calcareous matter of the matrix. 



Observations. — This species is based on two specimens of the ventral valve collected by 

 Prof. J. B. Woodworth. The generic reference is not entirely satisfactory as the pedicle open- 

 ing has not been clearly seen and tlie convex false area suggests the area of Micromitra (PI. II, 

 fig. 1) more than that of any Acrothele except A. spurri Walcott. 



The specific name is given in honor of Prof. J. B. Woodworth. 



Formation and locality. — Lower Cambrian: (326c) "Nahant limestone," Pulpit rock, Nahant, Essex County, 

 Massachusetts. 



Acrothele yorkensis Walcott. 



Plate LIX, figures 4, 4a-b. 



Acrothele yorlcensis Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 88-89, PI. IX, fig. 10. (Described 

 and discussed as below as a new species. Fig. 10 is copied in this monograph, PL LIX, fig. 4.) 



This species is represented by the casts, in the fine clay shales, of the exteriors of the valves, 

 and a few imperfect casts of the interior. The shell is large for a species of this genus. Ventral 

 valve moderately convex, with the apex in a specimen 9 mm. long 1.5 mm. from the posterior 

 margin. The general outline is subcircular, with the length and width approximately the 

 same. Dorsal valve with the apex marginal. The original convexity of the valves is unknown, 

 as all of the specimens are flattened in the shale. 



Surface marked by concentric ridges and striae of growth, and a few obscure, rounded, 

 radiating ridges. In addition there is a fine granulation of the type of that of A. coriacea, 

 the irregular, more or less inosculating, minute, rounded ridges having fine tubercles upon 

 them. 



A large ventral valve has a length and width of 11 mm. Other specimens of the same 

 size occur, although the average size is about 8 mm. Substance of the shell is unknown. 



One or two poor interiors of the ventral valve show a small visceral cavity, and rather 

 slender main vascular sinuses that appear to originate beside the pedicle opening. The latter 

 opens on the back slope of the apex and enlarges as it passes through the shell. The interior 

 of the dorsal valve shows a rather strong central ridge that extends beyond the center of the 

 shell, also a rather small cardinal scar on each side of the median ridge close to the posterior 

 margin. 



Observations. — This shell is of the general type of Acrothele matthewi (Hartt). It differs 

 in its larger size, and it is not probable that a species would occur in the inner Appalachian 

 trough which is present in the sediments near the margin of the Atlantic Basin. The fauna 



