346 CAMBKIAN BEACHIOPODA. 



This species is clearly distinguished from other described forms of the subgenus by its 

 highly arched pseudodeltidium and crenulated surface striae. 



An imperfect ventral valve from the upper portion of the Secret Canyon shale of the Eureka 

 district section, Nevada, has somewhat coarser striae than this species, but otherwise it appears to 

 be the same. Specimens from near Schellbourne, Nevada, appear to be identical with the Grand 

 Canyon form. A single imperfect ventral valve, from the Orr formation, 930 feet above the 

 base of the Upper Cambrian, in the House Range section of Utah, is very much like the specimen 

 from the Secret Canyon shale of Nevada and is tentatively placed with it under this species. 

 The Grand Canyon, Utah, and Nevada shells appear to be descendants of M. (P.) lahradorica 

 of the Lower Cambrian and its varieties in the lower portion of the Middle Cambrian of the 

 Cordilleran region. 



This form owes its specific name to the finely crenulated strise with which its outer surface 

 is characterized. 



FoKMATioN AND LOCALITY. — Upper Cambrian: (313) Limestone 0.75 mile (1.2 km.) east-northeast of McGill post 

 office, AATiite Pine County, Nevada. 



Middle Cambrian: (75) "Tonto" sandstone, near the water's edge, at the mouth of Kanab Canyon, where it 

 enters the Grand Canyon of the Colorado; (73b) sandstone in upper part of Tonto group, lower portion of Kwagunt 

 Valley, Grand Canyon of the Colorado; and (74) sandstone about 300 feet {91.4 m.) above the base of the Tonto group, at 

 the head of Nunhoueap Valley, Grand Canyon of the Colorado; all in Arizona. 



(313b) Limestone 3 miles (4.8 km.) north-northeast of Schellbourne, Schell Creek Range, ^Vhite Pine County, 

 Nevada. 



(13b) Sandstones of the Rome formation, northeast of Rhea Springs [Hayes, 1894, areal geology sheet], Roane 

 County, Tennessee. 



Specimens somewhat doubtfully referred to this species occur at the following localities: 



Upper Cambrian: (30]) About 950 feet (289.6 m.) above the Middle Cambrian and 2,450 feet (746.8 m.) below the 

 top of the Upper Cambrian near the base of the arenaceous shales and limestone forming le of the Orr formation [Wal- 

 cott, 1908f, p. 176J, on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House Range [Walcott, IQOSf , PL XIII], 

 Millard County, Utah. 



Middle Cambrian: (58) Shaly limestones in the upper beds of the Secret Canyon shale, east side of New York and 

 Secret canyons, Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



MicEOMiTEA (Paterina) etheridgei (Tate). 



Plate III, figm-es 10, lOa-c. 



Platyceras etheridgei Tate, 1892, Trans. Roy. Soc. South Australia for 1892, vol. 15, pt. 2, p. 184, PI. II, figs. 7a-c. (De- 

 scribed and discussed as a new species. The two specimens represented by figs. 7a-c are redrawn in this 

 monograph, PL III, figs. 10, lOa-b.) 



The original description by Tate follows : 



Shell small, broadly conical; apex obtuse, posterior, recurved, but not spii-al, usually protruding beyond the pos- 

 terior margin; apertm-e roundly oval, but iiTegular in outline, subtruncated and narrowed posteriorly. Surface orna- 

 mented with rather thick subimbricating concentric folds of growth, and ciu-ved beneath the apex coincidently with 

 the posterior margin; faintly radially wrinkled. 



This very common species exhibits considerable variability in the outline of the apertm-e, and the amount of back- 

 ward projection of the apex. The shells seem to have lived on iiTegular surfaces, and in some instances at least upon 

 trilobites. The proportionate measm-es of the basal diameter and height are as 4 to 3; the longest diameter observed is 6 

 mills. 



The exammation of the type specimen of this species shows that the shell substance is 

 corneous, that there is a true false area, and that the species is closely related to Micromitra 

 (Paterina) superia (Walcott). All of the specimens in the collection are ventral valves, and 

 these differ considerably in elevation, apparently owing to the upward arching of the posterior 

 margin beneath the false area. This characteristic distinguishes the species from M. (P.) 

 superba and allied forms. 



The specific name was given in honor of Dr. R. Etheridge, jr. 



Formation and locality.— Middle ? Cambrian: (315 [Tate, 1892, p. 184]) Limestone at Curramulka, Yorke 

 Peninsula, South Australia. 



