572 CAMBRIAN BEACHIOPODA. 



tube that extended on each side above the plane of the cardinal area so as to form a concave 

 plate similar to the posterior portion of the spondyhum in SyntropJiia (PL CII, fig. 6f). If 

 the sides of the plate were extended farther and united about the pedicle, a tube would result 

 similar to that of Obolella (PL LV, figs. If and Ih), and ultimately to that of Botsfordia (PL 

 LVII, figs. 4c and 4g). This plate in Dicellomus, while suggesting a spondylium, is more nat- 

 urally interpreted as the beginning of a pedicle tube. 



The interior of the dorsal valve has a well defined but very narrow area in one species, 

 D. politus (Hall), that is hollowed out so as to form a shallow groove on each side, apparently 

 for the reception of the short tooth of the ventral valve. No good posterior margins of the 

 valve could be found of the other species of the genus. The composite scar is large, and in 

 one shell it is subdivided into three small scars that were the points of attachment of the trans- 

 median and outside and middle laterals. The central and anterior lateral scars are arranged as 

 in Oholus; the centrals are large and located on the sides of the visceral area; the anterior 

 laterals are small and are located at the anterior end of the visceral area. 



Type. — Lingula polita Hall. 



Observations. — When referring to Dicellomus in 1899 I said [1899, p. 446]: 



When proposing that the genus Dicellomus include Obolella polita, Professor Hall [1873, p. 246] stated that the 

 grooving or emargination of the apices of both valves and the thickening of the edges of the shell on each side below 

 the apex, together with the form and character of the muscular impressions, would separate the species from Obolella. 

 Again, Messrs. Hall and Clarke [1892c, p. 72] gave a fuller description of Dicellomus politus, but owing to the poor 

 character of the mateiial they did not feel confident that it should be recognized as generically distinct from Obolella 

 chromatica. Material now in the collections of the United States Geological Survey clearly shows that Professor Hall's 

 provisional conclusion was correct, and that Dicellomus politus is generically distinct from Obolella chromatica. 



The presence of the large, composite, cardinal muscle scars in each valve suggests that 

 a search be made for a foraminal openmg, as in Obolella, Linnarssonella, and Acrotreta. No 

 trace has been found externally, and the interior of the ventral valve does not show evidence 

 of it, except in the jjedicle plate. Dicellomus appears to include characteristics of Obolus and 

 Obolella, and the presence of incipient dental plates, teeth, and dental grooves suggests an 

 articulate shell. 



, Dicellomus appalachia Walcott. 



Plate LIII, figures 4, 4a-g. 



Dicellomus appalachia Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 314. (Described and discussed as below as 

 a new species.) 



This shell has been identified with D. politus (Hall) in the prehminary studies of the genus 

 Dicellomus. I find that while it has the same type of shell structure and general form, it 

 differs in being less elongate proportionall}^ and in the details of the form and positions of the 

 muscle scars and vascular markings of the interior of the valve; it differs in the latter respect 

 from D. nanus (IMeek and Hayden). The outer thin layer has a dull, finely roughened surface 

 that is minutely punctate. When the outer layer is exfoliated the surface of the layer beneath 

 is highly pohshed and marked by exceedingly fine radiating and concentric stri^ and lines 

 of growth. The interior of the dorsal valve shows the large central muscle scars with the 

 minute anterolateral scars almost m contact with them. A composite scar (cl, PL LIII, 

 fig. 4d) shows the separate points of attachment of the transmedian (i), outside lateral (1), 

 and middle lateral (k) scars. The variation in the length of the visceral cavity of the dorsal 

 valve, as shown in Plate LIII, figures 4d to 4g, is very great, and it may be that figures 4f and 

 4g belong to another species. 



This species occurs in great abundance in both limestones and shales. It is the Appa- 

 lachian representative of the widely distributed D. politus of the interior of the continent, 

 hence the specific name. 



Formation and locality. — Middle Cambrian: (2z) Nolichucky (?) shale [Campbell, 1899, p. 3], near Shipley 

 Ferry, 0.75 mile (1.2 km.) northwest of Bethany Chapel, northeastern corner of the Roan Mountain quadrangle (U. S. 

 Geol. Survey); (2z') shales in the Honaker limestone [Campbell, 1899, p. 3], at Wallace switch, about 5 miles (8 km.) 

 northeast of Bristol, Bristol quadrangle (U. S. Geol. Survey); and (374o) sandy shale (Nolichucky?) [Campbell, 1899, 



