416 rnucEEDiNGS of tuk national museum. 



A cast of the interior of a ventral valve shows a clearly defined area 

 of medium leu<;th. It is divided midway by a cast of a narrow, strong" 

 l)edi(;le furrow, and on each side by Hexure lines situated about two- 

 thirds the distance from the ])edicle furrow to the lateral margin; a 

 few indistinct stria; cross the area parallel with its base. The area of 

 the dorsal valve as seen in a cast is well detined, and rather large; it 

 is marked by tine, transverse stria; of growth and indistinct (lexure 

 lines. A cast of the interior of the dorsal valve shows a trace of the 

 visceral cavity and a narrow median sei)tum. The only muscle scars 

 observed are the two umbonal scars in the ventral valve and the cen- 

 tral scars of the dorsal valve. 



Observations. — This very pretty little S])ecies oc(;urs in abundance in 

 the compact gray limestone associated with numerous fragments of 

 tiilobites and DiceUoinus nana. The more elongate forms strongly 

 resemble 0. (L.) perattenuaUis, which occurs in tlie Middle Cambrian 

 sandstones on the southern margin of the lUack llills. The species 

 differs, however, from the latter, in being more ovate, and in having 

 the dorsal valve more obtusely rounded i)Osteriorly. This species may 

 be also compared with 0. (L.) desideratus and (). (L.) manticulus. 



A small shell occurs in the St. Croix sandstone of the Upper Missis- 

 sippi region that appears to be identical with this species, both in its 

 typical form and in its comparatively wide range of variation. With 

 the somewhat abundiint supply of material from both the lUack llills 

 and Wisconsin, 1 am unable to determine any specilic differences that 

 are constant. 



Formation and locality. — Middle Cambrian, limestone beds in the 

 northern suburbs of Deadwood, IJlack llills, South Dakota, 



In the Upper Mississippi region the form identilied with this species 

 occurs in the yellow or bulf colored sandstones of the lower portion of 

 the Upper Cambrian fauna, at Winlield, and 4 miles north of VVin- 

 iield, Wisconsin, Kedwing and Keeds, Minnesota. 



A slightly larger shell occurs in the sandstone at Osceola Mills, 

 Wisconsin, that appears to be identical with those from a lower hori- 

 zon at Winlield, Wisconsin. In the elongate outline of the ventral 

 valve it resembles (). {Ij.)perattenuatas, but the data is insufficient to 

 identify it with that species. 



Tyj>c.s.— Nos. 27339-40, U.S.N.M. 



OBOLUS (LINGULELLA) SINOE, new species. 



General form broad ovate, with the ventral valve broadly subacumi 

 nate, and the dorsal valve broadly ovate. Valves moderately convex, 

 as far as can be determined from the series of shells pri^served in the 

 fine-grained sandstone. A ventral valve o jum. in length has a width 

 of 4.25 mm. A dorsal valve 4 mm. in length has an eijual width. 



The traces remaining of the exterior shell show it to have been 



