OBOLIDiE. 



421 



i 



Obolus woetheni Walcott. 

 Text figures 39A-D; Plate IX, figure 5e. 



C D 



FiGUKE 39.— Obolus wortheni Walcott A, Exterior of ventral valve (U. S. Nat. Mus. Cat. No. 51638b). 



B, Exterior of dorsal valve (U. S. Nat. Mus. Cat. No. 51C38c). C, Interior of ventral valve (U. S. Nat. 



Mus. Cat. No. 52431). D, Interior of ventral valve (U. S. Nat. Mus. Cat. No. 51638d). 



The specimens represented by figures 39A, 39B, and 39D are from Locality 54u, tliat represented by 

 figure 39C is from Locality 64t, both near Malade, Idaho. 



Obohis wortheni Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 63-64, PI. VII, fig. 17. (Described and dis- 

 cussed as below as a new species. Fig. 17, the type specimen, is copied in this monograph, PI. IX, fig. 5e.) 



General form subcircular, with the A'entral valve very obtusely acuminate and the dorsal 

 valve slightly transverse;- both valves slightly convex. Ventral valve with the beak at the 

 posterior margin, Avhich rises slightly from the general plane of the margin of the valve; the 

 minute beak of the dorsal valve is at the posterior margin. 



Surface marked by sharp, fine, concentric strife and fine imbricating lines of growth; on 

 some shells low, irregular, more or less obscure and interrupted radiating ridges occur. Shell 

 of medium thickness and built up of several layers or lamellae. The average diameter of the 

 valves is 3 mm. 



The interior of the ventral valve shows a short flat area divided midway by a narrow 

 pedicle furrow; the visceral area, which is about one-third the length of the valve, is shown 

 only in outline; the 

 main vascular sinuses 

 are strong and situ- 

 ated about midway 

 between the median 

 line and the lateral 

 margins of the valve ; 

 the surface outside 

 the visceral area in 

 both valves is marked 

 by fine co acentric 



furrows and large scattered punctte, much like those of Oholus ( Westonia) escasoni (Matthew) 

 (PI. XLIX, figs, la and laa). The interior of the dorsal valve has a short area with a broad 

 pedicle groove ; strong curved main vascular sinuses extend from beneath the area well toward 

 the front of the valve; they are subparallel to the margin and are situated about one-third the 

 distance from the margia to the median line of the valve; the visceral area is outlined in about 

 one-half the length of the valve; a narrow, deep sinus extends from each side of the anterior 

 end and then curves outward to the front margin (PI. IX, fig. 5e). 



Observations. — -This shell was at first thought to be the young of Oholus tetonensis Walcott, 

 but with the finding of a good series it was found to have a nearly circular ventral valve instead 

 of subacuminate as in 0. tetonensis, and it is less convex in the same character of matrix. In 

 form Oholus wortheni resembles Oholus discoideus (Hall and Whitfield) (PI. XVIII, figs. 6, 6a-d), 

 but it differs in being more circular in outline and in having a thinner shell. 



The specific name was given in honor of A. PI. Worthen. 



Formation and locality. — Upper Cambrian: (5a, 6c, 54t, and 54;w) Limestone about 250 feet (76 m.) above the 

 Middle Cambrian; (54x) limestone about 200 feet (60.4 m.) above the Middle Cambrian; and (54u) limestone about 

 100 feet {30.2 m.) above the Middle Cambrian; all in the St. Charles formation [Walcott, 1908a, p. 6], on the north 

 side of Twomile Canyon, near its mouth, 2 miles (3.2 km.) southeast of Malade, Oneida County, Idaho. 



Middle Cambrian: (54v) Limestone in the lower part of the Bloomington formation fV^'alcott, 1908a, p. 7], about 

 1,600 feet (488 m.) below the Upper Cambrian, on the south side of Twomile Canyon, near its mouth, 2 miles (3.2 km.) 

 southeast of Malade, Oneida County, Idaho. 



Obolxjs zetus (Walcott). 



Plate XXV, figures 3, 3a-c. 



Obolus (Linf/ulella) zetus W.iloott, 1898, Proc. U. S. Nat. Mus., vol. 21, p. 419. (Characterized and discussed as 

 below as a new species.) 



This is a small shell that has somewhat the general form of Oholus lamhorni (Meek). 

 It differs, however, in being more transverse across the front, in having a broad, shallow 



