444 ■ CAMBKIAN BKACHIOPODA. 



Of the remaining places of attachment of the muscles there are also plainly to be seen the ones lying in the neigh- 

 borhood of the area in the small shell, while they are lost, in the large shell, in the border of the excavation. 



The central depression in the large shell is always very shallow. 



Of the circulatory system are to be distinguished only the posterior ends of the principal vascular canals, under 

 the magnifying glass. 



The accessory vessels and the peripheral canals, on the contrary, are so feebly developed on all the observed speci- 

 mens that only indistinct traces are perceptible by reflected light. 



Formation and locality. — Upper Cambrian: OfeoZws sandstone at the following localities: (395)«' at Joa, near 

 Jegekcht, 12 miles {19.3 km.) east of Rcval; (395a) at Tihala, near Tegelecht; (395b) at Ilgast; (395c) at Asserien, 75 

 miles (121 km.) east of Reval; (395d) at Ontika, about 95 miles (153 km.) east of Reval and 30 miles (48 km.) west 

 of Narwa; and (395e) at Isenhof, 85 miles (137 km.) east of Reval; all [Mickwitz, 1896, p. 183] in the Government of 

 Esthonia, Russia. 



Obolus (Schmidtia) celatus (Volborth). 

 Plate XIV, figures 1, la-c. 



TJngula ovata Pander, 1830, Beitrage zur Geognosie des russischen Reiches, p. 59, PI. XXVIII, fig. 6a-b (not 



PI. Ill, fig. 23, referred to Obolus apoUinis). (Characterized in German.) 

 SchmidtiacelataYoLBOiiTii, 1869, Verhandl. Russ.-kais. min. Gesell. St. Petersburg, 2d ser., Bd. 4, pp. 209-212, PI. XVII, 



figs. 1-6. (Described and discussed in German as a new species.) 

 Schmidtia celata Volborth, Hall and Clarke, 1892, Eleventh Ann. Rept. State Geologist New York for 1891, p. 244, 



figs. 248 and 249. (No text reference. Figs. 248 and 249 are copied from Volborth [1869, PI. XVII, figs. 1 and 4, 



respectively].) 

 Schmidtia celata Volborth, Hall and Clarke, 1892, Forty-fifth Ann. Rept. New York State Museum for 1891, p. 560, 



figs. 248 and 249. (Copy of preceding reference.) 

 Schmidtia celata Volborth, Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 83, figs. 37 and 



38. (No text reference. Figs. 37 and 38 are copied from the figures given in the two preceding references.): 

 Obolus (Schmidtia) celatus (Volborth), Mickwitz, 1896, Mem. Acad. imp. sci. St.-Petersbourg, 8th ser., vol. 4, No. 2, 



pp. 159-163, PI. II, figs. 19 and 20. (Described and discussed in German; see below for translation.) 

 Obolus (Schmidtia) celatus orbiculatus Mickwitz, 1896, idem, pp. 163-165, PI. JI, figs. 21 and 22. (Described and dis- 

 cussed in German as a new variety.) 

 Obolus (Schmidtia) celatus prxcisus Mickwitz, 1896, idem, pp. 166-167, PI. II, figs. 37 and 38. (Described and discussed 



in German as a new variety.) 

 Oholus (Lingulella) celatus (Volborth), Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, PI. XXVI, figs. 1 and 2. (No 



text reference. The specimens represented by figs. 1 and 2 are redrawn in this monograph, PL XIV, figs, la 



and Ic, respectively.) 

 Obolus celatus orbiculatus (Mickwitz), Wiman, 1902, Bull. Geol. Inst. Univ. Upsala, vol. 6, pt. 1, No. 11, p. 62. (New 



locality mentioned in German.) 

 Obolus celatus orbiculatu.s (Mickwitz), Moberg and Segerbeeg, 1906, Medd. fran Lunds Geol. Faltklubb, Ser. B, 



No. 2 (Aftryck ur K. Fysiografiska Sallskapets Handl., N. F., Bd. 17), p. 65. (Mentions locality given in pre- 

 ceding reference, in Swedish.) 

 Obolus celatus orbiculatus (Mickwitz), WestergArd, 1909, Medd. fran Lunds Geol. Faltklubb, Ser. B, No. 4 (Aftryck 



m- K. Fysiografiska Sallskapets Handl., N. F., Bd. 20), p. 56. (Locality mentioned in Swedish.) 



The original description by Mickwitz follows : 



This species was first described by Volborth, who also gave us the fii-st knowledge of a part of the places of attach- 

 ment of the muscles. The first figure of a small shell is, however, found in Pander's above-cited work as TJngula ovata; 

 but this drawing lacks all internal characters with exception of the plate-shaped border of the shell, which Volborth also 

 mentions. Much more exact drawings of a whole series of Schmidtias (without names or description) are preserved 

 to us in a sheet among the papers left by Pander, which was kindly handed over to me by Professor Lahusen. The places 

 of attachment of the muscles, especially of the small shells, are in some of these figures present in full proportion, 

 but, to be sm'e, not disentangled as their individual constituent parts. Also on one specimen of a large shell are plainly 

 designated the principal and accessory vessels (which radiate inwardly). I have not utilized these drawings in any 

 present work, because my material is much more complete than that of Pander, and, moreover, the species described 

 by me could not with sufficient certainty be identified with the drawings in question. 



Diagnosis: Shells small, slightly arched; arching of the small shell slighter than that of the large shell. Outline 

 of the shells roundish, umbonal border of the large shell rectilinear. Surface of shell very smooth and lustrous; con- 

 centric striation very fine, in thickness somewhat irregular. Anterior (Stirn) and lateral borders thin, sharp margined; 

 those in the small shell usually somewhat thickened, with the umbonal borders lying in one plane. Ai'ea small, in 

 the largs shell slightly interrupted in the peduncular fm-row; in the small shell, in the splanchnoccelic part, deepened 



o Locality 395 is represented in tlie collections of tlie United States National Museum. 



