388 I'liOCKEDINdS OF THE NATIONAL MVHEVM. vol-xxi. 



species are lai<>;er tbau the ventral valve; one O.l'o iiini. long lias a 

 width of 0.75 mm., and anotlier 0.25 mm. long has a width of 7.25 mm. 



Casts of the interior of the ventral valve show a rather short area 

 that is not elearly defined in any of the si)eciniens. 



The cast of the i)edicle groove is narrow and merges into the cast of 

 the groove extending forward to the visceral area; the area is also 

 nnirked by flexure lines, and transverse stria' of growth. The area of 

 the dorsal valve is short in the one specimen showing it. The cast of 

 the visceral cavity on the ventral valve is clearly detiued by a rather 

 narrow ridge that is expanded anteriorly in what may have repre- 

 sented the lieaft shai)ed cavity. The parietal scar passes around in 

 front of the visceral cavity, and then a little backward to the main 

 vas(!ular sinuses. No traces of a median septum have been seen in 

 either vahe. 



The only traces of muscje scars observed are some irregnlar mark- 

 ings in the trapezoidal area, in which the central, middle, iind outside 

 lateral scars occur in the ventral valve. 



Obficr rat ions. — This somewhat i)eculiar si)e(;ies is associated with 

 JHcelloniKs politUN and Oholus namonna. The dorsal valves are clearly 

 distinct from any described form, approaching in some respects the 

 elongate dorsal valve of JAngiilepis acnmitiatd, dilferiug, however, in 

 being narrower and more elongate; the ventral valves api)ear to be 

 broader in proportion than the dorsal valve and approach O. matinalis 

 in outline, but are more elongate. 



Formation and locality. — Middle Cambrian, St. Croix sandstone, mid- 

 dle bed, at Eau Claire, Wisconsin. 



Type.—^o. 27300, U.S.N.M. 



OBOLUS PANDEMIA, new species. 



The external form of this species is much like that of <). watiiialis 

 Hall. It differs mainly in the characters of the interiors of the valves. 

 In the ventral valve the central ridge is elevated so as to be the most 

 pronounced feature; it not only fills up the space usually occui)ied by 

 the heart-shaped cavity, but rises much above the interior surface of 

 the shell. The trapezoidal areas and the dei)iessions occupied by the 

 nuiin vascular sinuses are seen only with difficulty. The essential 

 characters of the species are shown by the figures. 



Formation and locnliti/. — Middle Cambrian, argillaceous shales 

 embedded in the Rome sandstone, at Shooks Gap, Bays Mountain, 10 

 miles east of Kuoxville, Tennessee. 



Tyi>e.-No. 27301, U.S.X.M. 



OBOLUS ANCEPS, new species. 



General form broadly ovate, somewhat subcuneate. Valves moder- 

 ately convex. Surface marked by fine lines of growth and finer slightly 

 undulating concentric stria'. When the outer layer is exfoliated the 



