576 CAMBRIAN BEACHIOPODA. 



Lingulepis prima Meek and Hayden, 1865, Smithsonian Contrib. Knowl., No. 172, Paleontology Tipper Missouri, pt. 1, 



p. 3, PI. I, figs. 2a-b. (Described and discussed as a new species. The specimen represented by figs. 2a and 



2b is redrawn in this monograph, PL Lll, fig. 2h.) 

 Obolella ? polita Hall, 1867, Trans. Albany Inst., vol. 5, pp. 112-113, PL I, figs. 17-21. (Text and figures copied from 



Hall, 1863, pp. 133-134, PL VI, figs. 17-21.) 

 Dicellomus polita Hall, 1873, Twenty-third Ann. Kept. New York State Cab. Nat. Hist., p. 246. (Changes generic 



reference.) 

 Obolella polita (Hall), Whitfield, 1880, U. S. Geog. and Geol. Survey Rocky Mtn. Region, Rept. Geology and Resources 



Black Hills of Dakota, by Newton and Jenney, pp. 339-340, PL II, figs. 12 and 13. (Described and discussed. 



The specimen represented by fig. 13 is redrawn in this monograph, PL LII, fig. 2g; fig. 12 is not.) 

 Obolella polita (Hall), Waloott, 1886, Bull. U. S. Geol. Survey No. 30, p. 111. (Discussed.) 

 Obolella polita (Hall), Hall and Clakke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 72 and 73, PL II, 



figs. 37^1. (Discussed as a species of Dicellomus, but the use of that term is held in abeyance until the species 



shall have been proven generically distinct from Obolella. Figs. 40 and 41 are copied from Hall, 1863, PL VI, 



figs. 20 and 21.) 

 Dicellomus politus (Hall), Walcott, 1899, Men. U. S. Geol. Siu-vey, vol. 32, pt. 2, pp. 443 and 446, PL LX, figs. 4 and 4a. 



(Discussed, establishing the generic distinctness of Dicelloirais. The specimens represented by figs. 4 and 4a 



are redrawn in this monograph, PL LII, figs. Ij and Ic, respectively.) 

 Dicellomus politus (Hall), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 313. (Copy of text referred to in preceding 



reference.) 

 Dicellomus politus (Hall), Walcott, 1905, idem, pp. 316-318. (Described and discussed as below, although some 



changes and additions have been made.) 

 Dicellomus politus (Hall), Gkabau and Shimee, 1907, North 

 American Index Fossils, vol. 1, pp. 189-190, fig. 224, 

 p. 190. (Described. The three figures in fig. 224 are 

 copied from Hall, 1863, PL VI, figs. 17, 20, and 21.) 



General form ovate, with the ventral valve 

 subacuminate; biconvex, the ventral valve usuaBy 

 more convex than the dorsal. Surface smooth as 

 the specimens usually occur, but a few examples 

 K ^ show that there is a thin outer layer covered with 



FiGTJEE m.-Dicdiomus politus (Hall). A, toterior of ventral fine papillsB and puncta3 that appear to be placed 



valve (fsp, pseudospondvlium or pedicle plate; i, transmediau ^ , . i j_* • i 



scars; j, anterolateral scars; V, visceral cavity; vs, main vas- On Very tme, mOrC Or ICSS mOSCUlatmg ridges. 



cular sinuses). B, Posterior portion of the interior of a ConCCntric linCS of grOWth of Varying strength 



ventralvalve.sliowingastronglydeveloped area (fsp, pseudo- j.i j. j> i j.i • 



spondylium or pedicle plate; a, inside section of area; f, ShOW OU the OUtcr SUllace, alSO On the VariOUS 



flexure line; a', outside section of area). inner layers or lamcllaj. When the outer layer is 



The specimens represented by figures 49A and 49B are from g^foliated Or WOm off by attrition of the Sand, the 

 Locality 79s, Upper Cambrian "St. Croix sandstone" at Hud- . . ' 



son, Wisconsin (0. s. Nat. Mus. Cat. Nos. 51917a and 51917b, surfacc o-f the inner layer is marked by radiatmg 



respectively). The same figures are reproduced on page 314. ^^^^^ ^^^ COUCCntric Imes of growth that are a 



marked feature of the species. The radiating strise may be uniform in size, or with 4 to 6 very 

 fine strise between two stronger elevated striae. Inner surface smooth to the eye, but a strong 

 lens shows that it is finety punctate, with exceedingly fine, irregular, raised lines all over the 

 surface. The intermediate layer is more coarsely and iiTegularly punctate. 



The shell is thick, and is built up of a thm, scabrous, outer layer, a middle layer made up 

 of several highly polished lamellas, and a thin inner layer. The numerous inner lamellse are 

 oblique to the outer surface over much of the shell, and thus give it thickness. Shell sub- 

 stance calcareocorneous. The usual length of the ventral valves from Wisconsin is about 

 6 mm., width 5 mm., with the same width and length for the 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 indi- 

 cate a short cardinal area that extends out on the cardinal margins of the shell and appears 

 to merge into the margins. On each side of the pedicle furrow a thin plate or shelf (sh) extended 

 into the valve on the plane of the margins of the shell. Some of the casts indicate that these 



