448 CAMBRIAN BRACHIOPODA. 



only in those shells whose characters, upon the whole, are strongly pronounced. It is self-evident that such fine 

 particularities are only perceptible by strong magnification in reflected light; they could not be distinctly reproduced 

 without impairing the value of the figures. 



The secondary vessels, which radiate inwardly, vary essentially both in their course and in the manner of their 

 ramification. In the large shell, as a rule, they ramify rectilinearly and perpendicularly to the principal vessels and 

 throw off short branches to both sides; only those which lie more anteriorly curve toward the anterior border and 

 open between the principal vascular furrows into the peripheral canal. 



The accessory vessels run out entirely different in another specimen. Here they fork shortly after branching off 

 from the principal vessels and take their course to the posterior part of the shell. Nothing is to be seen of an opening 

 into the peripheral canal of the anterior border. Lastly, another shell, where these organs leave the most indistinct 

 traces, shows an iiTegular confusion of treelike ramified canals, the directions of which seem to alter by every turning 

 of the shell under the lens. 



The accessory vessels of the small shell, which radiate inwardly, show similar differences. As a rule, the accessory 

 vessels issuing from the central part of the principal vessels, turn anteriorly immediately after their exit from the 

 latter, and are slightly cm'ved in the shape of an S and hardly ramified. Only those lying at the extreme back bend 

 into the angular spaces of the brachiocoele, which are formed by the parietal band; the accessory vessels, however, 

 which ramify from the anterior, inwardly bent «nds of the principal vessels, run rectilinearly posteriorly. Another 

 specimen, however, shows the central accessory vessels directed more posteriorly and very strongly ramified. 



What significance these essential deviations have in the systematic aspect can not, however, be established at 

 present. No correlation has hitherto been obtained. 



The secondary outwardly radiating vascular traces correspond, in all examined specimens, in regard to their 

 arrangement and course. Branching off perpendicularly to the principal vessels, the straight, short traces fork before 

 opening into the peripheral canal. 



As in 0. celatus, the parietal band may be followed as a fine furrowed swelling, and where perceptible traces are 

 lacking it is often established through the difference in the character of the surface of the splanchnocoele and 

 brachiocoele. 



This form owes its specific name to the massiveness of its shells. 



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

 Jegelecht, 12 miles {19.3 km.) east of Reval; (395a) at Tihala, near Jegelecht; (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. 192] in the Government 

 of Esthonia, Russia. 



Obolus (Schmidtia) obtusus Mickwitz. 



Plate XIV, figures 3, 3a-c. 



Obolus (Schmidtia) ohtusus Mickwitz, 1896, M^m. Acad. imp. aci. St.-P6tersbom-g, 8th ser., vol. 4, No. 2, pp. 167-171, 

 PI. II, figs. 23, 24, 33, and 34. (Described and discussed in German as a new species; see below for translation.) 



OboliLs (Schmidtia) obtusus longus Mickwitz, 1896, idem, pp. 171-172, PL II, fig. 25. (Described and discussed in 

 German as new variety. ) 



Obolus (Schmidtia) obtusus acutus Mickwitz, 1896, idem, pp. 172-173, PL II, fig. 26. (Described and discussed in 

 German as a new variety. ) 



Obolus (Schmidtia) obtusus latus Mickwitz, 1896, idem, pp. 174-175, PL II, fig. 27. (Described and discussed in Ger- 

 man as a new variety.) 



Obolus (Schmidtia) obtusus minutus Mickwitz, 1896, idem, pp. 17.5-177, PL II, figs. 29 and 30. (Described and discussed 

 in German as a new variety. ) 



Obolus (Schmidtia) obtusus ellipticus Mickwitz, 1896, idem, pp. 177-178, PL II, figs. 31 and 32. (Described and dis- 

 cussed in German as a new variety.) 



Obolus (Schmidtia) obtusus extenuatus Mickwitz, 1896, idem, pp. 178-179, PL II, figs. 35 and 36. (Described and dis- 

 cussed in German as a new variety.) 



Obolus obtusus (Mickwitz)?, Wiman, 1902, Bull. Geol. Inst. Univ. Upsala, Bd. 6, pt. 1, No. 11, p. 63, PL III, fig. 12. 

 (New locality mentioned in German.) 



Obolus obtusus (Mickwitz)?, Moberg and Segeeberg, 1906, Medd. fran Lunds Geol. Faltklubb, Ser. B, No. 2 (Aftryck 

 ur K. Fysiografiska Sallskapets HandL, N. F., Bd. 17), p. 65. (Locality mentioned in Swedish.) 



Obolus obtusus (Mickwitz)?, Westerg.Ird, 1909, Medd. fran Lunds Geol. Faltklubb, Ser. B, No. 4 (Aftryck ur K. 

 Fysiografiska Sallskapets HandL, N. F., Bd. 20), p. 57. (Locality mentioned in Swedish.) 



The original description by Mickwitz follows : 



This extraordinary species, whose principal characteristics consist in the peculiar character of the area which is 

 predominantly prolonged, consists of a large number ot shells, which do not essentially deviate in outline and size. 

 All of these forms are rare; of some of them only one was found; in consequence, their cospecificity could not be proven 



a Locality 395 is represented in tlie United States National Museum collections 



