NO. 1395. CAMBRIAN BRA CHIOPODA— WALCOTT. 313 



could be found of the other species of the genus. The composite scar 

 is hirg-e, and in one shell it is subdivided into three small scars that 

 were the points of attachment of the transmedian, outside, and middle 

 laterals. The central and anterior lateral scars are arranged as in 

 (>hoh(s\ the centrals are large and located on the sides of the visceral 

 area; the anterior laterals are small and located at the arterial end of 

 the visceral cavity. 



Oheer vat tons. — When referring to DiceUomus.^ in 1809, I said:" 



When proposing that the genus Dicellomus^ include OboleUa poliia, Professor Hall 

 stated that the grooving or einargination 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 

 OboleUa. Again in 1892 Messrs. Hall and Clarke '' gave a fuller description of 

 Dkelknnus politus, but owing to the poor character of the material they did not feel 

 confident that it should ])e recognized as generically distinct from OboleUa chromaiica. 

 Material now in the collections of the Geological Survey clearly shows that Professor 

 Hall's provisional conclusion was correct, and that Dkellomus politus is generically 

 distinct from OboleUa chroniatica. 



The ])resencc of the large, composite, cardinal muscle scars in each 

 valve suggests that a search be made for a foramina! opening, as in 

 Ohohjllii, Lhindrsmudla., and Acrotreta. No trace has been found 

 externally, and the interior of the ventral valve does not show evi- 

 dence of it. D'lceUom uh appears to include characteristics of Oholus 

 and OhoJelJa., and on the presence of the incipient dental plates, teeth, 

 and dental grooves an articulate shell is suggested. In Me ehina 'prima 

 a step further is taken in the development of the spondylium and in 

 the approach toward articulating brachiopods. 



The stratigraphic range of DlceUoinns is from the Middle Cambrian 

 to the latter part of the Upper Cambrian. Its geographic distribu- 

 tion includes the Appalachian area in Tennessee and Alabama, the 

 Mississippi basin from Wisconsin and Minnesota on the north to 

 Oklahoma on the south and South Dokota, Montana, and Utah on 

 the west. The Arizona locality appears to be an extension to the 

 southwest of the upper Mississippi Valley species, D. iJoUtus. The 

 only form from Europe is suggested by the unidentified shell from the 

 Paradoxides forch.liainerl zone of Anduarum, Sweden. 



The species now referred to the genus are: 



Dicellomus appalaclila^ Middle Cambrian. 



D. jyarvus^ Middle Cambrian. 



I), jyectenoides, Middle Cambrian. 



D. politus^ Middle Cambrian. 



D. species undetermined. Middle Cambrian. 



D. nanu.^^ Upper Cambrian. 



«Mon. U. S. Geol. Survey, XXXII, Pt. 2, 1899, p. 446. 

 ''Twenty-third Ann. Kept. New York State Cab. Nat. Hist., p. 246. 

 <'Pal. New York, VIII, Pt. 1, p. 72. 



