54 SMITHSONIAN MISCELIvANEOUS COLLECTIONS VOL. 53 



Genus MICKWITZIA Schmidt [1888, p. 24] 

 MICKWITZIA OCCIDENS, new. species 



Plate 7, Figure i 



There are only crushed and broken specimens of this shell. One 

 of these shows that the apex of the ventral valve was a little above 

 the posterior margin of the shell, very much as in Mickwitzia pre- 

 tiosa. The outline of the valves appears to have been ovate to sub- 

 circular, with the ventral valve moderately convex. The shell is 

 phosphatic or chitinous and built up of three principal layers. The 

 outer layer is thin and thickly set with minute pustules or granules 

 that give the surface a roughened appearance. When the outer layer 

 is exfoliated, which is usually the case, the middle layer presents a 

 smooth, shining surface that is marked by a few concentric striae 

 and numerous fine radiating striae, between which many very minute 

 punctae occur. The inner layer shows minute, irregular, serpentine, 

 rounded ridges, perforated by vertical canals or punctae. An inte- 

 rior of a ventral valve shows the lines of adyance of the antero- 

 lateral muscle scars. The largest shell indicated on the surface of 

 the siliceous shale has a length and width of 12 mm. 



This species and the generic reference is based on the character 

 of the apex of the ventral valve and the structure and character of 

 the shell. 



Formation and Locality. — Lower Cambrian : ( i ) Near the base 

 of the section, about 5,500 feet (1,676.4 m.) below the top of the 

 Lower Cambrian, in shaly indurated sandstones, one mile (1.61 km.) 

 east of the Saline Valley road, and 2 to 3 miles (3.22 to 4.83 km.) 

 east-northeast of Waucoba Springs, Inyo County, California. (2) 

 Sandstones on small hill in the salt flat one mile {1.61 km.) northeast 

 of Silver Peak Mill, Silver Peak quadrangle ( U. S. G.S.), Esmeralda 

 County, Nevada} 



MICKWITZIA PRETIOSA, new species 



Plate 7, Figure 2 



This species is founded on a single specimen of a ventral valve. 

 It has a length of 7 mm. ; width, 6.5 mm. Outline subcircular, 

 slightly convex; apex curved over toward the posterior margin and 

 projecting beyond it. False area short and obscure. Surface 

 marked by radiating, raised lines, that at the front margin show six 



' Where there is more than one locality, the one from which the type speci- 

 mens come is italicised. 



