302 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



margin, and in the still larger all traces of the perforations have dis- 

 appeared, and true spines occur usually six on each side and two or 

 four on the posterior margin, as in Pccdciiiiiias traiisitans [pi. 34, 

 figs. 5, 6, and 7]. 



Fragments of thoracic segments associated with the cephalon indi- 

 cate [figs. 10, II, pi. 31] that the pleural lobes were broad, and that 

 the pleurae of each segment continued outward and curved gently 

 backward, narrowing gradually to a sharp point, as in IJ'\ walcot- 

 tanns [pi. 30, figs, i, 10-12]. 



Surface very rarely preserved owing to the maceration and com- 

 pression of the specimens in the fine arenaceo-argillaceous shale. 

 The few traces of it left indicate that it was similar to that of W. 

 zvalcottanus in having an irregular network of fine, irregular ridges 

 over the greater part of the surface ; this is shown for the latter 

 species by figs. 12, 13, pi. 31, and for ]V. halli by figs. 10, 11, pi. 31. 



Dimensions. — The largest fragment of the cephalon in the col- 

 lection indicates a complete cephalon 60 mm. in length with a width 

 of no mm. and a dorsal shield based on the proportions of JV. 

 cvaicottaiiiis of about 150 mm. in length. 



Reference to the younger stages of growth of this species may be 

 found under the description of the development of the individual of 

 the Mesonacidte (pp. 236-243). 



The specific name is given in honor of Prof. James Hall. 



Formation and Locality. — Lower Cambrian : in the upper por- 

 tion of the Montevallo formation at the following localities: (56c) 

 ■argillaceo-arciiaccoiis slialcs about 1,000 feet {305 m.) northeast of 

 tozvii of Helena, on roadside just north of Buck Creek; and (164c) 

 4 miles (6.4 km.) south of Helena on road to Montevallo; both in 

 Shelby County, Alabama. 



WANNERIA WALCOTTANUS (Wanner) 



Plate 30, Figs. 1-12; Plate 31, Figs. 12 and 13 



Olciiclliis (Hohiiia) zcalccltanits Wanner, 1901, Proc. Washington Acad. 

 Sci., Vol. 3. pp. 267-269, pi. 31, figs. I. 2 ; pi. 32 figs. 1-4. (Described and 

 discussed as a new species. The specimens represented by i, 2, and 3, 

 pi. 32 are redrawn in this paper, pi. 30, figs. 6, 5, and 7 respectively. 

 The specimen represented by figure 2, pi. 31, is redrawn in this paper, 

 pi. 31, fig. 12.) 



This is one of the largest sj^iecies of the Mesonacidar, and like 

 Callaria briii^i^-eri (Walcott), C. crosbyi VValcott, and C. biirri Wal- 

 ■cott, occurs in the u]>]:)er portion of the North American Lower Cam- 



