390 



CAMBKIAN BKACHIOPODA. 



V 



Obolus dolatus (Sardeson). 

 Text figures 35A-C. 



Lingula dolata Sardeson, 1896, Bull. Minnesota Acad. Nat. Sci., vol. 4, p. 95, PI. VI, fig. 12. (Described as a new 

 species.) 



Only the exterior of the valves of this species is known. The outline of the valves is much 

 like tliat of Oholus matinalis (Hall) (PI. VIII), and it does not differ more in this respect from 



FiGXJEE 35. — Obolus dolatus (Sardeson). A, Exterior of a large, crushed ventral valve, the type specimen. 



B, Exterior of an elongate dorsal valve. C, Surface ridges on the front part of another dorsal valve, X14. 



The specimens represented are all from Locahty 339, near Stillwater, Minnesota, in beds which are reported by 



Sardeson to be the Lower Ordovician Oneota dolomite. 



that species than specimens of 0. matinalis differ from each other. The cliief specific distinc- 

 tion is m the fine, narrow, sharp, slightly undulating, slightly inosculating concentric ridges; 

 the spaces between the ridges are about twice the width of the ridges. This surface is some- 

 what similar to that on the interspaces between the beaded ridge of Oholus (Mickwitzella) 

 siluricus (PI. XV, fig. Ic). 



It may be that when specimens of the mterior of the valves are found other differences 

 from 0. matinalis will be noted. Oholus dolatus appears to be a Lower Ordovician representative 

 of the widely distributed 0. matinalis. 



Formation" and locality. — Lower Ordovician : (339 [Sardeson, 1896, pp. 95 and 96]) from beds referred by Sardeson 

 to the Oneota dolomite, Stillwater, Washington County, Minnesota. 



I Obolus eichwaldi Mickwitz. 



Obolics eichwaldi Mickwitz, 1896, Mem. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 154-155, PI. II, 

 figs. 15a-d. (Described and discussed in German as a new species.) 



Of this species Mickwitz had only the posterior portion of a dorsal valve and some small 

 fragments. He states that the essential characteristics which distinguish it from 0. triangu- 

 laris Mickwitz are the much finer, more regular, concentric striation and the somewhat irregu- 

 lar, undulating, radial ribs, which give tlae surface a somewhat wavy appearance through their 

 varying strength and occasional interruption. Mickwitz speaks of the circular form of the fehell, 

 but the lines of growth on the fragment illustrated (PL II, fig. 15a) indicate a somewhat trans- 

 verse shell. He also mentions peculiarities in the area of the dorsal valve. The student is 

 referred to tlie elaborate description of Mickwitz for further details. 



Mickwitz states that 0. eichwaldi is one of the rarest species of the Oholus sandstone. 



The specific name was given in honor of C. E. von Eichwald. 

 Formation and locality. — Upper Cambrian: (395 [Mickwitz, 1896, p. 155]) Oholus sandstone at Joa, near 

 Jegelecht, 12 miles (19.3 km.) east of Reval, Government of Esthonia, Russia. 



Obolus elegans Mickwitz. 

 Plate XV, figure 3. 

 Oholus elegans Mickwitz, 1896, M6m. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 157-158, PI. II, fig. 

 18. (Described and discussed in German as a new species.) 



Only fragments were found of "this most delicate species of the Oholus sandstone." The 

 shell appears to have been roundish and rather strongly convex, judging from one of the larger 

 fragments, which does not exceed 5 mm. in diameter. 



