26o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Width of axial lobe, anterior segment 6.5 



Width of axial lobe, seventeenth segment 3. 



Width of axial lobe, twenty-seventh segment 1.5 



Width of pleural lobe, anterior segments 6.5 



Width of pleural lobe, seventeenth segment I. 



Pygidinm: 



Length about 1.75 



Width at front 1.5 



The preceding description is based on specimens compressed in 

 an arenaceous shale that has had sufficient distortion to flatten the 

 dorsal shield and widen it. The normal form of the cephalon is 

 probably nearest to that of the cephalon represented by fig. 5. The 

 largest dorsal shield found indicates a total length of 126 mm. 



Obsen'atioiis. — This species is one of the most primitive known 

 to me. The form of the anterior lobe of the glabella is primitive, 

 and its twenty-eight segments with posterior eleven so very simple 

 indicate a closer approach to annelidian progenitors than any of its 

 associates in the Lower Cambrian, Georgian, fauna ; it has three 

 more thoracic segments than Meson ads vermontana (Hall) [pi. 26, 

 fig. i] and the posterior eleven are more primitive in form. 



Nevadia wecksi differs from Elliptocephala asaphoides [pi. 24] 

 in so many ways that it is not necessary to describe them. The 

 points of generic similarity are in the cephalon where the general 

 characters are similar ; in the thorax where the segments are of the 

 same type back to the rudimentary segments ; and the pygidium ap- 

 pears to be similar, although relatively much smaller in N. zvecksi. 



This species was identified and named Holmia zvecksi, new species, 

 and the name used in the Barrel Spring geological section [Walcott, 

 1908, pp. 188-189], and in the Waucoba Springs section [Idem, 

 pp. 186-187]. 



The specific name is in recognition of the excellent work of Mr. 

 F. B. Weeks, formerly of the United States Geological Survey. 



Formation and Locality.' — Lower Cambrian : Silver Peak 

 Group in hard arenaceous shales at the following localities : ( if) 

 ill No. 12 of the Barrel Spri)ig section [JValcott, igo8c, p. i8q], j 

 miles (4.8 km.) northeast of Barrel Spring, zvkich is 10 miles (16 

 km.) south of the tozvn of Silver Peak; and (174b) 10 miles (16 

 km.) northwest of Silver Peak on ridge north of Red Mountain; 

 both in Esmeralda County, Nevada. 



^ The type locality is given in italics, when there is more than one locality. 

 The locality numbers in heavy-face type are the numbers assigned to the speci- 

 mens in the collections of the United States National Museum. 



