NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 317 



OboleUa pulita Hall and Clarke, Pal. New York, VIII, 1893, Pt. 1, pp. 72, 73, 



pi. II, fig?. 37-41. 

 DiceUomus polUns Walcott, Mon. U. 8. Geol. Survey, XXXII, 1899, Pt. 2, 



p. 446, pi. L.>;, figs. 4, 4a. 



General form ovate, with the ventral valve subacuniinate; biconvex, 

 the ventral valve usuallly more convex than the dorsal. Surface 

 smooth as the specimens usually occur, but a few examples show that 

 there is a thin outer layer covered with fine papilte and punctge that 

 appear to be placed on very tine, more or less inosculating ridges. 

 Concentric lines of growth of varying strength show on the outer sur- 

 face, also on the various inner layers or lamella?. When the outer 

 layer is exfoliated or worn off by attrition of the sand, the surface of 

 the inner layer is marked by radiating strite and concentric lines of 

 growth that are a marked feature of the species. The radiating striae 

 may be uniform in size, or with from 4 to 6 very fine striae between 

 two stronger elevated stria?. Inner surface smooth to the eye, but a 

 strong lens shows that it is finely punctate, with exceedingly fine, 

 irregular, raised lines all over the surface. The intermediate layer 

 is more coarsely and irregularl}^ punctate. 



The shell is thick and is built up of a thin, scabrous, outer layer, a 

 middle kwer made up of several highly polished lamellfe, and a thin 

 inner layer. The numerous inner lamellae are oblique to the outer 

 sui-face over much of the shell, and thus give it thickness. Shell 

 substance calcareo-corneous. The usual size of the Wisconsin shells 

 is about 6 mm. long, width 5 mm., for ventral valve, and same width 

 and length for dorsal valve. Specimens from the Black Hills average 

 a trifle larger than those from Wisconsin. 



Ventral valve with a uniform convexity over the central portions, 

 from which the surface slopes gently to the margins. Apex nearly, 

 if not quite, marginal. Casts of the interior indicate a short cardinal 

 area that extends out on the cardinal margins of the shell, and that 

 appears to merge into the margins. On each side of the pedicle fur- 

 row a thin plate or shelf extended into the vah^e on the plane of the 

 margins of the shell. Some of the casts indicate that these plates had 

 a projecting boss or tooth that articulated in a rude manner with the 

 depressions on the posterior, flattened, inner margin of the dorsal 

 valve. If this interpretation is correct, the teeth and plates corre- 

 spond to the teeth and dental plates of the articulate brachiopods, and 

 the plates mark the beginning of a spondylium. The posterior mus- 

 cles, transmedian and anterior laterals were undoulUedly attached 

 within the area of the composite scar. The visceral area and poste- 

 rior portions of the main vascular sinuses are well outlined, but no 

 nmscle scars or details of the vascular S3^stem have been preserved in 

 the material studied. 



Dorsal valve most convex back of the center. Apex marginal. The 

 Proc. N. M. vol. xxviii— 04 21 



