80 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Observations. — Dcarhornia clarki is one of the simple or rudi- 

 mentary forms of the Siphonotretidfe. It differs from Siphonotreta 

 in the absence of an area and a siphonal or pedicle tube, in having 

 the pedicle opening on the umbo in advance of the beak, and in its 

 calcareous shell. The circular pedicle aperture without an exterior 

 furrow, the absence of a well-defined area on the ventral valve, and 

 its calcareous shell distinguish it from Trcinatoholns and Schizam- 

 bon. The form and position of the pedicle opening suggest Discin- 

 opsis, but the interiors of the valves are very dissimilar in the two 

 genera. It may be that with the discovery of good exteriors of the 

 ventral valve of Trematoholus cxcclsis Walcott (see below) that 

 species will be found to have a circular pedicle opening of the same 

 character as that of Dcarhornia clarki, but from the similarity of the 

 cast of the interior of the ventral valve of the former species to that 

 of Trematoholus kciiipaiiiiiii (Matthew) [1897, P- /o] it is referred 

 to Trcmatohohis. 



The generic name is taken from Mount Dearborn, which was 

 named after Gen. Henry Dearborn, and the specific name is given 

 in recognition of Dr. William B. Clark's work on the paleontology 

 of Maryland. 



Formation and Locality. — Middle Cambrian : Lower portion 

 of the Yogo limestone, in the canyon of the north fork of the 

 Dearborn River, in the eastern part of the Lewis and Clark Forest 

 Reserve, Montana. 



Genus TREMATOBOLUS Matthew [1893, p. 276] 

 TREMATOBOLUS EXCELSIS, new species 



Plate 8, Figure 8 



Shell transversely oval in outline, with both valves obtusely acumi- 

 nate. Ventral valve strongly convex, with the minute beak at the 

 posterior margin above a low area; the slope from the highest 

 point of the valve, a little back of the center, is greatest toward the 

 beak and nearly uniform to the front and sides of the valve. Pedicle 

 opening unknown, as no exterior or cast of the exterior of the valve 

 occurs in the material collected ; two casts of the interior show the 

 cast of the foramen at about the same position as in Trematoholus 

 insignis Matthew [1893, p. 276] and other species of the genus. 

 Dorsal valve slightly more transverse than the ventral and about two- 

 thirds as convex ; a very slight median flattening occurs at the ante- 

 rior margin that extends back on the valve — nearly to the beak in 

 some specimens ; otherwise the convexity is distributed as in the 

 ventral valve. 



