754 CAMBRIAN BEACHIOPODA. 



BiLLINGSELLA DICE Walcott. 



Plate CI, figures 8, 8a-d. 



BilUngsella dice Walcott, 1905, Proc. U. S. iSfat. Mus., vol. 28, p. 234. (Characterized essentially as below as a new 

 species.) 



In general form and surface this shell is related to B. romingeri (Barrande). It differs 

 from the latter in its strong dental plates, which form a rudimentary spondylium. All the 

 characters known are illustrated. 



Formation and locality. — Lower Ordovician? : (319f) Drift bowlder of sandstone found near St. Albans, Franklin 

 County, Vermont. The lithologic character of the matrix in which this species was found points to the arenaceous 

 limestones of the Phillipsburg formation just north of the boundary between the United States and Canada as the 

 source of the bowlder'. 



BiLLINGSELLA EXPORRECTA (Linnarsson) . 



Plate LXXXVIII, figures 1, la-1. 



Orthis exporrecta Linnarsson, 1876, Bihang till K. svensk. Vet.-Akad. Handl., Bd. 3, No. 12, pp. 12-13, PL II, figs. 

 13-19; PI. Ill, figs. 20 and 21. (Described and discussed in English as a new species.) 



Orthis expoirecta Linnarsson, Kayser, 1883, China, by Richthofen, vol. 4, p. 35. (Discussed in German in the descrip- 

 tion of "0. linnarssoni.") 



Orthis exporrecta Linnarsson, Wallerixts, 1895, LTndersokningar ofver Zonen med Agnostus hvigatus i ^'esterg6tland, 

 p. 66. (Localities mentioned in Swedish.) 



BilUngsella exporrecta (Linnarsson), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 234-236. (Described and 

 discussed- essentially as below.) 



Shell transversely subsemicircular, with the liinge line usually shorter than the greatest 

 width. In a few examples it is a trifle longer; the cardinal angles are obtusely angular (PI. 

 LXXXVIII, fig. le). The proportions between the length and breadth vary, as shown by 

 the figures. The ventral valve is considerably more convex than the dorsal: The ventral 

 valve is evenly rounded, while the dorsal usually has a flattening of the median area and some- 

 times a slight, broad sinus. The surface is marked by numerous radiating costse and very 

 fine concentric striae between the costse, and a few visible lines of growth. The costse are 

 rounded, usually, but not always, broader than the interspaces, and continuous from the 

 umbo to the front and lateral margins; they appear to be quite regular, although varying 

 much in size, and increasing somewhat irregularly by interpolation of new costse. This usually 

 occurs near the umbo, but may occur near the middle of the valve. The largest ventral valve 

 in the collection has a length of 9 mm. and a breadth of 11 mm. Dorsal valve: Length, 7 mm.; 

 breadth, 10 mm. 



Cardinal area of ventral valve high and overhanging the posterior margin. Delthyrium 

 large and triangular in outline; deltidium short, concave, imperforate, and marked bj"- fine, 

 transverse striae of growth. Cardinal area of dorsal valve short and inclined slightly forward; 

 it is divided midway by a broad delthyrium, which is divided b}^ a strong cardinal process. 



In the interior of the ventral valve strong main vascular sinuses extend well forward 

 toward the front margin. The cast of the umbonal cavity opposite the delthyrium shows 

 a tripartite division, the center of which is the area of the path of advance of the diductor 

 muscle impressions, and those on each side appear to be the posterior portion of the main 

 vascular sinuses. The anteri&r edge of the cavity is marked by a sharp, elevated, forward- 

 arcliing ridge that extends to the teeth on either side; the dental plates appear to have 

 extended to the bottom of the valve and to have bounded the sides of the triangular umbonal 

 cavity opposite the delthyrium. 



The interiore of the dorsal valve are beautifully preserved. The cardinal process, crura, 

 crural plates, area, anterior and posterior adductor muscle scars, and vascular sinuses are 

 very distinct; the dental sockets are shallow, but clearly shown. The great variation in the 

 size, form, and position of the adductor scars is well illustrated by the figures. In figure If 

 there appear to be two lateral supporting septa to the thickening before the crural plates that 



