OBOLIDiE. 419 



Obolus triangularis Mickwitz. 



Obolus triangularis Mickwitz, 1896, Mem. Acad. imp. aci. St.-P6teisbourg, 8th ser., vol. 4, No. 2, pp. 14&-147, PL II, 



figs. 7-9. (Described and discussed in German as a new species; see below for translation of diagnosis.) 

 Obolus triangularis iiwrnatus Mickwitz, 1896, idem, pp. 148-149, PI. II, figs. 10-12. (Described and discussed in 



German as a new variety.) 

 Obolus triangularis Mickwitz, Moberg and Segbeberg, 1906, Medd. fra,n Lunds Geol. Faltklubb, Ser. B, No. 2 (Aftryck 



ur K. Fysiografiska Sallskapets Handl., N. P., Bd. 17), p. 65. (New locality mentioned in Swedish.) 

 Obolus triangularis Mickwitz, WestbrgIrd, 1909, Medd. frta Lunds Geol. Faltklubb, Ser. B, No. 4 (Aftryck ur K. 



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



The original description by Mickwitz follows: 



Shells moderately large, slightly grooved, thin, brittle; outline of shell subtriangular, posterior border of shells 

 rectilinear, not winged; surface slightly shiny, concentric strise in fine ribs, which converge to the border of the shell, 

 either diverging or confluent, running somewhat irregularly, separated by deep, rounded grooves; radial strise 

 dwindling, indefinite. Area large, splanchnocoelic part broad, peduncular groove wide, parallel bordered, opening 

 toward the inside of the shell. Central groove of the large shell heart-shaped to subpentagonal, slightly swelled. 

 Median septum and lateral septa perceptibly developed. Sinus of small shell reaching nearly to the area, corneous 

 processes ridge-shaped. Combined central places for the attachment of muscles of the large shells are not prolonged 

 into grooves posteriorly. 



Mickwitz states that the shells of this species are very thin and brittle and that in the 

 enormous mass of loose Oiolus conglomerate that he examined at Joa not one thick-shelled 

 specimen was found. The shell also occurs alone at Reval under the Obolus conglomerate, 

 and they are so delicate that they can not be taken from the matrix, although it is a very loose, 

 fine-grained sandstone. The thick shells of the Oholus conglomerate have a more confused 

 ornamentation and a somewhat wider form. The concentric ribs of the typical form have 

 the best development near the center of the shell, where they run together and are dotted ia 

 places with mdistinct knots formed by the intersection of the irregularly developed radial 

 strise with the concentric strise. He calls attention to the broad pedicle groove of the ventral 

 valve, which is of the same type as that of 0. panderi Mickwitz, and describes m detail the 

 interior marldngs of the valves. 



The variety inornatus does not appear to me to be sufficiently distinct to be entered as a 

 variety. 



Formation and locality. — TTpper Cambrian: (395g) Obolus conglomerate at Joa, near Jegelecht, 12 miles 

 {19 .3 km.) east of Reval; (3951) Obolus conglomerate at Ilgast; and (3361) sandstone below the Obolus conglomerate 

 proper, at Reval; all [Mickwitz, 1896, p. 147] in the Government of Esthonia, Russia. 



(9d) Obolus sandstone at Jaggowal, about 20 miles (32 km.) east-southeast of Reval, Government of Esthonia, 

 Russia. 



(SlOn) Obolus sandstone? [Moberg and Segerberg, 1906, p. 65], in Dalarne, Province of Kopparberg, Sweden. 



^ Obolus volboethi Mickwitz. 



Obolus volborthi Mickwitz, 1896, M6m. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 155-157, PL II, 

 figs. 16-17. (Described and discussed in German as a new species.) 



This species is separated from 0. eicJiwaldi Mickwitz on the difference in the character 

 of the surface. Of this species only tln-ee fragments have been found. According to Mick- 

 witz, it is distinguished from 0. eichwaldi by its oval outline and pronounced inclination of 

 the beak. He gives a detailed description and comparison, to which the student is referred. 



The specific name was given in honor of Dr. A. von Volborth. 



Formation and locality. — Upper Cambrian: (395 [Mickwitz, 1896, p. 157]) Obolus sandstone at Joa, neat 

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



