PATEKINID^. 353 



The surface of both valves is marked by clearly defined, regular, concentric strife that are 

 slightly crenulated by strise radiating from the beak outward to the margin. The concentric 

 strife show 10 in a distance of 1 mm. and 8 at the center of the dorsal valve. 



Observations. — Nothing is known of the interior or the areas of either valve. The clearly 

 defined concentric strife, the thick corneous shell, and the long convex pseudodeltidium dis- 

 tinguish the species from all others of the genus known to me, with the possible exception of 

 Micromitra (Paterina) labradorica swantonensis (Walcott). In the latter species, however, the 

 stria3 are less regular, and present quite a different appearance under the magnifier. In the 

 original description [Walcott, lSS4b, p. 19] the comparison was made with Kutorgina cingulata 

 (Billings) as to certain points of resemblance between M. (P.) prospedensis and. the young 

 shells of K. cingulata. 



This form owes its specific name to its occurrence on Prospect Mountain in the Eureka 

 district, Nevada. 



Formation and locality. — Lower Cambrian: (Im and Ip) Limestones of No. 2 of the Silver Peak group, Barrel 

 Spring section [Walcott, 1908f, p. 189], about 2.5 miles (4 km.) south of Barrel Spring and 0.5 mile (0.8 km.) east of 

 the road, in the extreme southeastern corner of the Silver Peak quadrangle (U. S. Geol. Survey), Esmeralda County; 

 and (62) arenaceous shales just above the Prospect Mountain quartzite, in a stratigraphic position similar to that of the Pioche 

 shale [Walcott, 1908/, p. 184], at the top of Prospect Mountain, Eureka district [Hague, 1892, Atlas'], Eureka County; both 

 in Nevada. 



y Micromitra (Pateeina) sTissiNGENSis-(Dwight). 



Plate III, figures 1, la-e. 



Kutorgina stissingensis Dwight, 1889, Am. Jour. Sci., 3d ser., vol. 38, pp. 145-147, PL VI, figs. 5-8. (Described and 



discussed as a new species; see below for copy of paragraph on p. 146. The specimens represented by figs. 5, 6, 



and 8 are redrawn in this monograph, PL III, figs. Ic, 1, and la, respectively.) 

 Kutorgina stissingensis Dwight, 1891, Trans. Vassar Brothers' Inst, for 1887-1890, vol. 5, pt. 2, p. 105, figs. 5-8, p. 



108. (Described and discussed as a new species.) 

 Iphidea stissingensis (Dwight), Schtjchekt, 1897, Bull. U. S. Geol. Survey No. 87, p. 234. (Merely changes generic 



reference.) 

 Iphidella stissingensis (Dwight), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 308. (Merely changes generic 



reference.) 



All the specimens of this species known to me are more or less compressed in an argillaceo- 

 arenaceous shale and impure limestone. As far as can be determined the ventral valve is 

 obtusely conical, with the apex slightly incurving. 



Dwight [1889, p. 145] in his original description mentions the presence of a distinct false 

 area on the ventral valve, and I find in the two specimens traces of the area, also a wide, low, 

 sUghtly convex pseudodeltidium. 



The dorsal valve appears to have been slightly convex and to have resembled closely the 

 general form and proportions of the same valve of Micromitra (Paterina) labradorica (Billings). 

 One somewhat crushed specimen shows a very low false area and a rather broad, low pseudo- 

 deltidium somewhat like that of M. (P.) labradorica. It is too imperfect, however, for detailed 

 description or illustration. 



The surface of both valves is covered with very fine, sharp, slightly crenulated striae and 

 fine ridges of growth, crossed by very fine radiating strise and fine radiating undulations of 

 the same type as those in Micromitra sculptilis (Meek). The concentric strife cross the false 

 areas and pass over the pseudodeltidium. Shell substance corneous. 



Dwight [1889, p. 146] describes the surface markings in detail as follows: 



The concentric ridges are somewhat wavy as seen under a strong magnifier; they are semicircular; a number of 

 those lying nearest to the front margin run out along the upper part of the lateral margins, but the remainder and 

 larger number terminate in regular order along the cardinal border. In front of the central portions of the shell the 

 concentric ridges, which number about 12 to 15 to a millimeter, are regularly concentric; but nearer to the beak the 

 number and the irregularity greatly increase. At a point about one- third the length of the shell from the beak there 

 are 25 or more to the millimeter; as the radiating plications are numerous in this part, there is caused a complexity of 

 curves, which under a powerful magnifier produces the effect of elegant and delicate basketwork. The radiating 

 undulations are very irregular in position and number; they are not thoroughly continuous from the beak in specimens 

 62667°— VOL 51, pt 1—12 23 



