358 



CAMBRIAN BRACHIOPODA. 



MicEOMiTRA (Paterina) williardi Walcott. 

 Text figures 30A-E; Plate II, figures 9b-c. 



Iphidella major Walcott (in part), 1905, Proc. U. S. Nat. Mus., vol. 28, p. 304. (Specimens now referred to M. (P.) 

 williardi were included with the specimens representing M. (P.) major when this description was written.) 



Micromitra (Paterina) williardi Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 60, PL VII, fig. 7. (Described 

 and discussed as below as a new species. Fig. 7 is copied in this monograph as fig. 30A.) 



Ventral valve subconical, with the apex over the posterior third of the subcircular margin 

 of the valve ; false area narrow but clearly defined by a rather sharp angle on the cardinal slopes 



that breaks the curvature of the 

 shell a short distance from the 

 margin of the pseudodeltidium; 

 pseudodeltidium broad, con- 

 vex, with its lower margia 

 broadly arched so as to leave 

 a space between it and the gen- 

 eral plane of the margin of the 

 shell. Some specimens of the 

 pseudodeltidium are uniformly 

 rounded, in others there is a 

 narrow groove extendmg from 

 the apex to the base, and on 

 some a very narrow faint ridge 

 is indicated. 



Dorsal valve slightly con- 

 vex, transverse, and slightly 

 rounded at the cardinal margin. 

 No traces of a false area or 

 pseudodeltidium have been 

 observed. 



The cast of the interior of the apex of the ventral valve shows a small apical callosity with 

 two radiating grooves extending upward toward the front lateral margin of the shell. 



Surface marked b}^ very fine, strong, concentric, elevated striae that in a specimen 10 mm. 

 in diameter show seven elevated strife in a distance of 1 mm. ; the elevated striae are crossed 

 by very fine transverse striae; the elevated striae cross the false area parallel to its base and arch 

 over the pseudodeltidium. 



A ventral valve 10.5 mm. in diameter has a height of 2.5 mm. 



Ohservations. — This species is closely related to Micromitra (Paterina) swperha (PL II). It 

 differs in having a longer pseudodeltidium, more finely elevated striae on the surface, and a more 

 sharply elevated apex to the ventral valve. It is the Lower Cambrian representative of M. (P.) 

 superha. 



The specific name is given in honor of Mr. T. E. WUliard, who collected the type specimen. 



Formation and locality. — Lower Cambrian: (17b) Rome ("Montevallo") formation, 4 miles (6.4 km.) south 

 of Helena; and (56c) Rome (" Montevallo") formation, 1.125 miles (1.8 hm.) northeast of Helena; both in Shelby County, 

 Alabama. 



riGUEE 30.— Micromitra (Paterina) williardi Walcott. A, A', A", Top, side, and back of a partly 

 flattened ventral valve (U. S. Nat. Mus. Cat. No. 514S2a). B, Pseudodeltidium with strong 

 lines and ridges of growth (U. S. Nat. Mus. Cat. No. 514S2b). C, Pseudodeltidium with 

 median furrow (U. S. Nat. Mus. Cat. No. 51482c). D, A small dorsal valve (U. S. Nat. Mus. 

 Cat. No. 51482d). E, Enlargement of the outer surface near the front margin of a ventral 

 valve, X 15 (U. S. Nat. Mus. Cat. No. 51482e). Figure 30A represents the type specimen. 

 The specimens represented are from Locality S6c, 1.125 miles (1.8 km.) northeast of Helena, 



Shelby County, Alabama. Figure 30A is copied from Walcott [1908d, PI. VII, flg. 7). 



