314 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



DICELLOMUS APPALACHIA, new species. 



Tliis .shell hiis heon identitied with I). poUtux in the preliminary 

 studies of the genus Dicellomus. I find that while it has the same 

 type of shell structure and g"eneral form, it differs in being less elon- 

 gate proportionally, 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. na)mi<. The outer thin layer has 

 a dull, slightly roughened surface that is minutely punctate. When 

 the outer layer is exfoliated the surface of the layer beneath is highly 

 polished and marked by exceedingly fine radiating strife and concen- 

 tric stria* and lines of growth. The interior of the dorsal valve shows 

 the larger central muscle scars with the minute antero-lateral scars 

 almost in contact with them. A composite scar shows the separate 

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

 scars. The variation in the length of the visceral cavity of the dorsal 

 valve is very great. 



This species occurs in great aljundancc in both limestones and shales. 

 It is the representative of the widely distributed I). })olitu8 of the 

 interior of the continent. ' ' 



Foriii(it!(i/i and locality. — Middle Cambrian. Limestones beneath 

 the upper shale of the Cambrian section at many localities in Jefferson 

 and Hawkins counties; limestone layers in Conasauga shale, 2 miles 

 south of Coal Creek, Anderson County; in shale on road from Rogers- 

 ville to Dodson Ford; limestone at Bull Run, Copper Ridge, 11 miles 

 northwest of Knoxville, southwest of Maynardsville, and 5 miles 

 southeast of (xreenville. Tennessee. 



Coosa shale. CoAvan Creek and Cedar Bluff', Cherokee County, Mur- 

 phees Valley, and Blountsville Valley, Alabama. 



DICELLOMUS NANUS Meek and Hayden. 



Oboh'/la nana jMeek and Hayden, Proc. Acad. Nat. Sci. Philadelphia, 2d ser., 

 V, 1861, p. 435.— Billings, Pal. Foss., I, 1862, p. 67.— Hayden, Am. Jour. 

 Sci., 2d ser., XXXIII, 1863, p. 73. — Meek and Hayden, Pal. Upper Mis- 

 souri, Pt. 1, 1864, p. 4, pi. I, figs. r>(i-d. — Whitfield, U. S. Geog. and Geol. 

 Surv. Eocky Mountain Region, 1880, p. 340, pi. ii, figs. 14-17.— Hall and 

 Clarke, Pal. New York, VIII, 1892, Pt. 1, p. 69. 



Dicellomus nanus Walcott, Mon. U. S. Geol. Survey, XXXII, 1899, p. 447, pi. 

 lx, figs 3, 3«-d. 



The principal characters of this species are the same as Dicellomus 

 polifm. The two forms diff'er exteriorly in D. nanus being more con- 

 vex on the umbones and less elongate. The interior of the ventral 

 valve shows a less elongate visceral area, a relatively larger composite 

 muscle scar. The interior of the dorsal valve of D. nanus has a much 

 larger composite muscle scar, a broader visceral area with the central 

 muscle scar farther from the median line. 



