OBOLID^. 407 



the ridges are little more than elevated striae at the aperture, becoming gradually coarser over 

 the central and anterior portions of the valve, where there are from five to seven ridges in a 

 distance of 1 mm.; toward the apex there are more than double the number in the same 

 distance. 



Oholus ohscurus has a larger and more rounded shell than 0. chinensis and is also distiu- 

 guished by the strong, concentrically striated surface, which resembles that of Micromitra 

 (Paterina) labradorica orientalis (Walcott) , from which it differs in the form of the shell. It 

 differs from Oholus (Acritis) antiquissimus (Eichwald) (PI. XIII) in having the concentric striae 

 or ridges more regular and very rarely bifurcating. Oholus {Acritis?) rug atus IV aicoii (PL 

 XIII) has much stronger concentric ridges. The surface of 0. nundina Walcott (PI. XI) is 

 marked by irregularly spaced, rounded, concentric ridges, with rounded depressions between 

 them, instead of the V-shaped depressions and regulai-ly spaced ridges of 0. ohscurus. Oholus 

 ohscurus has a large strong shell of the general type of Oholus rhea Walcott (PI. IX) of the 

 Middle and Upper Cambrian of Wisconsin. 



The material upon which this species was based [Walcott, 1905a, p. 326] was more or 

 less fragmentary, and the specific name was chosen because of the obscure relations of the 

 species. 



Formation and locality. — Middle Cambrian: (C63) Sandy shale near the base of the Kiulung group [Blackwelder, 

 1907a, p. 37 (3d paragraph), and fig. 8a {bed 32), p. 29], 3.5 miles {5.6 km.) southwest of Yenchuang, Sintai district. 

 Shantung; and (C75) limestone near the base of the Kichou formation [Willis and Blackwelder, 1907, p. 143], 4.5 

 miles (7.2 km.) south of Wutaihien, Shansi; both in China. 



Obolus? palliatus Barrande. 



Plate XV, figure 5. 



Obolvs? palliatus Barrande (in part), 1868, Faune silurienne des environs de Hof, en Bavifere, pp. 104-105, fig. 65 

 (not fig. 64, referred in this monograph to Obolus f bavaricus). (Described and discussed in French as a new 

 species. Fig. 65 is copied in this monograph, PI. XV, fig. 5.) 



Ohjl'.is ? palliatus Barrande (in part), 1868, Neues Jahrb. fiir Mineralogie for 1868, p. 693, unnumbered plate, fig. 65 

 (not fig. 64, referred in this monograph to Obolus f bavaricus). (Text and figures copied from preceding reference.) 



This shell is described from the external characters, and on this account was referred 

 provisionally to the genus Oholus. The author states that the form is transverse, beak obtuse 

 and the convexity of the shell as preserved is greatest near the beak. The substance of the 

 shell appears to be like that of the associated Lingula. It is ornamented with fine concentric 

 striae, wliich tend to group themselves into bands on some specimens. On Barrande's figure 

 radiating striae are shown, but are not mentioned in the text. Two specimens have a length 

 of 13 mm., with their widths respectively 16 and 18 mm. 



Formation and locality. — Passage beds between the Upper Cambrian and the Ordovician: (303c [Barrande 

 1868a, p. 105]) Suburbs of Hof, Bavaria, Germany. 



Obolus pandemia Walcott. 



1 



Plate IX, figures 3, 3a-d. 



Obolus pandemia AValcott, 1898, Proc. U. S. Nat. Mus., vol. 21, p. 388. (Characterized as below as a new species.) 

 The external form of this species is nauch like that of 0. matinalis (Hall) (PI. VIII). It 

 differs mainly in the characters of the interiors of the valves. In the ventral valve the 

 central ridge is elevated so as to be the most pronounced feature. It not only fills up the 

 space usually occupied by the heart-shaped cavity, but rises much above the" interior surface 

 of the shell. The trapezoidal areas (c) and the depressions occupied by the main vascular 

 sinuses are seen only with difficulty. The essential characters of the species are shown by 

 figures on Plate IX. 



Formation and locality. — Middle Cambrian: (10a) Sandy layers of the Rome formation, in west railroad cut 

 through Shocks Gap, in Bays Mountains, 10 miles (16.1 km.) southeast of Knoxville [Keith, 1895, areal geology sheet], 

 Knox County, Tennessee. 



