324 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



.striiK. The inner and outer .surfuces of theshells have very line punctse, 

 but no traces of punctte penetrating- throug-h the shell have been 

 observed. 



The largest venti'al valve has a lengtli of 22 mm., width 23 mm.; 

 dorsal valve, length 17 mm., width 18 mm. Shell substance calcare- 

 ous, rather thick on the umbonal region and thin towards the margins. 



Ventral valve subacuminate, moderatel}^ convex; on some of the 

 larger shells a broad mesial fold and arching of the frontal margin are 

 present; apex terminates at the margin above a triangular opening or 

 delthyrium; area small and within the plane of the margins of the 

 valve; it is attached to the bottom of the valve and divided midway by 

 a very slight pedicle groove; the margins of the delthyrium-like open- 

 ing sustain an angle of about 45" to the plane of the valve, and the 

 reversed area within is at its center nearly on the plane of the valve; 

 the anterior margins of the area extend upward to meet the margins 

 of the shell, thus forming a reversed arched area. The interior of the 

 valve is marked l)y radiating, shallow, narrow furrows that vary in 

 number and strength in different shells; just in front of the area on 

 each side of the median line oval depressions occiu* which correspond 

 in position and shape to the diductor muscle scars of the ventral valve 

 of the Orthidffi, and they are so interpreted. Traces of the adductor 

 scars are found on each side of the median line. Dorsal valve obtusely 

 acuminate, about as convex as the ventral valve; apex marginal on 

 the broad, low, median arch of the posterior margin; no traces of an 

 area have been observed. A broad, shallow mesial sinus and project- 

 ing front margin occur on adult shells. The position of the anterior 

 and posterior adductor muscle scars is indicated, also the umbonal 

 thickening in which the diductor muscles were probably attached, as 

 there is no trace of a cardinal process. Radiating furrows like those 

 in the ventral valve are strongly marked in adult shells. 



(>})Hei'vatUni!<, — ^eliuclievtina camhria is a shell that, as far as 1 know, 

 is unique. In its exterior form it suggests a biconvex Orthoid genus, 

 and this likeness is further increased by the ilabelliform scars of the 

 ventral valve; the smooth surface and reversed areas suggest Ellaivla., 

 while the interior imibonal portion of the ventral valve points to 

 Qaehecia. It may be said that Schuchertina is a type derived from 

 some eai'ly Cambrian form allied to Quebecia and that its general 

 aspects affiliate it with the Orthida^. 



Formation, and locaJ/ity. — Middle Cambrian limestone. Thirteen 

 miles south of Neihart, 1 mile below divide on road to Woolsey post- 

 office, Montana. 



OBOLUS ACADICA, new species. 



Only interiors of the dorsal valve of this species occur in the col- 

 lections made by Mr. S. W. Loper on McNeils Brook in 1900. The 



