6o SMITHSONIAN miscellane;ous collections vol. 53 



MICROMITRA (PATERINA) WILLIARDI, new species 



Plate ", Figure 7 



Iphidclla iitajor Waecott (in part), 1905, Proc. U. S. National Museum, 

 XXVIII, p. 304. (Specimens now referred to M. (P.) ivilliardi were 

 included with the specimens representing M. (P.) major when this 

 description was written.) 



A'entral valve stibconical, with the apex over the posterior third 

 of the subcirctilar 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 ; pseudodeltidiimi broad, convex, with its 

 lower margin broadly arched, so as to leave a space between it and 

 the general plane of the margin of the shell. Some specimens of 

 the pseudodeltidium are uniformly rounded, in others there is a 

 narrow groove extending from the apex to the base, and on some a 

 very narrow faint ridge is indicated. 



Dorsal valve slightly convex, 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 by very fine, strong, concentric, elevated striae. 

 A specimen 10 mm. in diameter shows seven of these elevated striae in 

 a distance of i mm. ; the elevated striae are crossed by very fine trans- 

 verse 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. 



Observations. — This species is closely related to Microiiiitra 

 (Pafcriiia) siipcrha (Walcott) [1897&, p. 711]. It chfifers 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.) snperha. 



The associated fossils are Oboliis sniithi (see p. 62), JJliiiaiiclla 

 shclbyciisis (see p. 100), Microniitro (Pafcrina) major (Walcott) 

 [1905^, p. 304], and numerous fragments of two or three species of 

 OIciiclliis. 



FoK.MATiON AND LocALiTv. — Lowcr Cambrian: ]\Iontevallo argil- 

 laceous shale (1)4 miles (6.44 km.) south of Helena; and (2) .sj 

 mile (.40 kilt.) northeast of Helena; both in Shelby County, Ala- 

 bama. 



