626 CAMBRIAN BRACHIOPODA. 



opening that penetrates through the shell, but whether the opening is continued as a tube is 

 unknoAvn; a false area is slightly indicated on the slope between the pedicle aperture and the 

 gently upward-arching posterior margin. Dorsal valve depressed in some examples so as to be 

 slightly concave between the minute beak at the posterior margin and the front; in other 

 examples it is flat and in some slightly convex. 



Surface of both valves marked by concentric lines of growth and low ridges from which 

 spine bases extend across to the front edge of the next ridge, very much as in Siphonotreta 

 unguiculata (Eichwald) (PI. LXXX) ; the spine bases are short and scattered on the posterior 

 parts, numerous and elongated toward the front and lateral margins in the adult shells. 



The average diameter of the valves is from 3 to 4 mm. Substance of shell probably 

 calcareocorneous ; it is ' calcareous in the specimens, but this appears to have been the result 

 of replacement of the original shell by impure calcite, which has also replaced the substance 

 of associated trilobite tests. 



The generic reference is doubtful, as nothing is known of the interior of either valve. The 

 circular pedicle opening distinguishes the species from Schizambon typicalis Walcott, wliich has 

 a similar spinose surface (PI. LXXXIV, fig. Id), and relates it to Siphonotreta. I think that 

 with full knowledge of this species it wUl be found to belong to an imdefined genus, as it is not 

 probable that Siphonotreta was developed so early in Cambrian time. 



This form owes its specific name to the doubtfulness of the generic reference. 



Formation and locality. — Lower Cambrian: (Iv) Shales of No. 3 of the Silver Peak group, Barrel Spring 

 section [Walcott, 1908f, p. 189], 3 miles (4.8 km.) north of Valcalda Spring and 4 miles (6.4 km.) west-northwest of 

 the Drinkwater mine, Silver Peak quadrangle (U. S. Geol. Survey), Esmeralda County, Nevada. 



Siphonotreta unguiculata (Eichwald). 

 \ 



Plate I.XXXI, figures 6, 6a-e. 



Crania ungmculata Eichwald, 1829, Zoologia specialis, vol. 1, p. 274, PI. IV, fig. 3. (Described in Latin as a new 



species.) 

 Crania sulcata Eichwald, 1829, idem, p. 274, PL IV, fig. 4. (Described in Latin as a new species.) 

 Terehratula unguiculata Eichwald, 1840, Jour. Natur- und Heilkunde Medizinischen Akad. St. Petersburg, pts. 1 and 



2, pp. 138-140 (pp. 24r-26 in second part of one edition). (Described in German.) 

 Terehratula unguiculata Eichwald, 1842, Die Ui-welt Russlands, Hit. 2, p. 145, PL IV, figs. 2a-b. (Mentioned in 



German.) 

 Siphonotreta unguiculata (Eichwald), de Verneuil, 1845, Geologic de la Russie d'Eiu-ope, by Murchison, de Verneuil, 



and de Keyserling, vol. 2, pt. 3, pp. 286-287, PL I, fig. 13. (Described in French.) 

 Siphonotreta unguiculata (Eichwald), Kutorga, 1848, Verhandl. Russ.-kais. min. Gesell. St. Petersbiu-g for 1847, No. 



12, pp. 264 and 284, PL VI, figs, la-d, 2a-c, 3a-c. (Described in German on p. 264.) 

 Siphonotreta unguiculata var. a Kutorga, 1848, idem, p. 284, PL VI, figs. 4a-e. (The description of this variety 



may be incorporated with that of the species on p. 264; it is only mentioned, however, in the description of 



PL VI, which is on p. 284.) 

 Siphonotreta unguiculata var. Kutorga, 1848, idem, pp. 264-265, PL VI, figs. 5a-d. (Characterized in German.) 

 Siphonotreta unguiculata var. ;- elongata Kutorga, 1848, idem, p. 265, PL VI, figs. 6a-b. (Characterized in German 



as a new variety.) 

 Siphonotreta unguiculata (Eichwald), Davidson, 1853, British Fossil Brachiopoda, vol. 1, Introduction, No. 3, PL 



IX, figs. 261-265, 269 and 270. (Mentioned on p. 133. Fig. 269 is copied from Kutorga, 1848, PL VI, fig. la.) 

 Siphonotreta unguiculata Eichwald, 1860, Lethsea rossica, ancienne periode, vol. 1, sec. 2, pp. 915-916. (Described 



and discussed in French.) 

 Siphonotreta unguiculata (Eichwald), Schmidt, 1861, Archiv ftir Naturkunde Liv-, Ehst-, und Kurlands, 1st ser.,Bd. 



2, p. 218. (Localities mentioned.) 

 Siphonotreta unguiculata (Eichwald), Quenstedt, 1871, Petrefactenkunde Deutschlands, Abth. 1, Bd. 2, Brachio- 



poden, p. 674, PL LXI, figs. 23-27. (Discussed.) 

 Siphonotreta unguiculata (Eichwald), Davidson, 1877, Geol. Mag. new ser., dec. 2, vol. 4, footnote and figs. A and B, 



p. 14; p. 15, PL II, figs. 7-11. (Described and discussed. Fig. 9 is copied in this monograph, PL LXXXI, 



fig. 6c.) 

 Siphonotreta unguiculata (Eichwald), Zittel, 1880, Handbuch der Palaontologie, Bd. 1, Abth. 1, p. 666, figs. 489a-d. 



(No text reference. Figs. 489a-d are copied from Davidson, 1853, PL IX, figs. 265, 264, 262, and 261, respec- 

 tively.) 

 Siphonotreta unguiculata (Eichwald), Walcott, 1884, Mon. U. S. Geol. Survey, vol. 8, p. 16, PL I, fig. 8. (Discussed. 



Fig. 8 is copied from Davidson, 1877, PL II, fig. 9.) 



