OBOLID^. 435 



Obolus apollinis Eichwald, de Verneuil (in part), 1845, G^ologie de la Russie d'Em-ope, by Muuchison, de Verneuil, 

 and de Keyserling, vol. 2, pt. 3, pp. 290-292. (Described and discussed in French. The reference includes 

 Obolus { Mickwitzella) siluricus, Obolus (Acritis) antiquissimus, Obolus apollinis, and Obolus apollinis ingricus; 

 Obolus apollinis being figured.) 



Aulonotreta polita Kutorga (in part), 1848, Verhandl. Russ.-kais. min. Gesell. St. Petersburg for 1847, No. 12, pp. 

 279-282 (not PI. VII, figs. lOa-f). (Described and discussed in German as a new species. The text includes 

 Obolus (Mickwitzella) siluricus, Obolus apollinis ingricus, and Obolus apollinis, the last species being figured.) 



Obolus siluricus Eichwald, 1859, Lethaea rossica, ancienne periode, Atlas, PI. XXXVII, figs. 6 and 7a-b. (The text 

 reference accompanying these figm'es is in the following citation.) 



Obolus siluriciis Eichwald, 1860, Lethsea rossica, ancienne p&iode, vol. 1, sec. 2, pp. 927-928. (Described and dis- 

 cussed in French.) 



Obolus siluricus Eichwald, Schmidt, 1861, Archiv fiir Naturkunde Liv-, Ehst-, und Km-lands, 1st ser., vol. 2, p. 218. 

 (Mentioned in German.) 



Obolus siluricus Eichwald, Schmidt, 1881, M^m. Acad. Imp. Sci. St.-P6tersbourg, 7tli ser., vol. 30, No. 1, p. 17. (Local- 

 ity mentioned in German.) 



Lingula cf. davisi Salter, Schmidt, 1881, idem, fig. 5, p. 17. (Mentioned in the text.) 



Obolus siluricus Eichwald, Mickwitz, 1892, Melanges g^ol. et pal^ontol. tires du Bull. Acad. imp. sci. St.-P6tersbom-g, 

 vol. 1, p. 60. (Mentioned in German.) 



Obolus (Thjsanotos) siluricus (Eichwald), Mickwitz, 1896, M6m. Acad. imp. sci. St.-Petersbom'g, 8th ser., vol. 4, No. 

 ■ 2, pp. 195-199,. PI. Ill, figs. 1-9. (Described and discussed in German; see below for translation. Fig. 9 is 

 copied in this monograph, PI. XV, fig. lb.) 



Mickwitz [1896, pp. 195-199] describes the species as follows: 



Shells large, vei-y flatly arched, thin, brittle. Outline of shells round, somewhat broadened. Posterior borders of 

 shells rectilinear, not winged. Tip of the beak of large shell slightly curved inwardly. Smiace of shell very lustrous, 

 like varnish. Concentric strife in sharp-edged ribs, which are separated by flat troughs. These ribs run parallel to 

 the posterior borders of the gi'owth lamellae and therefore always become shorter toward the anterior borders of the 

 same. Growth lamellae prettily fringed at the anterior borders; the concentric striae are continued on the fringes. 

 Area large, quite smoothly striate, slightly excavated at the base of the splanchnoccelic part. Pleurocoelic part of the 

 area extends far into the lateral borders of the shell. Traces of the pseudo-area lamellse approach the borders of the 

 peduncular gi-oove, and converge into the tip of the beak. Peduncular gi-oove large, deep, roundly hollowed, inwardly 

 strongly broadened. Thickening of the shells very slight. Central groove of the large shell indefinitely flat. Corneous 

 processes and sinus of the small shell strongly receding. Median swelling of the small shell broad and flat, divided at 

 its anterior half by a broad median groove. Principal vascular traces broad and flat, separated by a median swelling 

 into two sharp-cut fm-rows. Places of attachment of the central muscles of the small shell large, cii'cular; those of the 

 umbonal muscles very coarsely longitudinally striate. 



Observations: The large shell of this species reminds one in form of 0. apollinis quenstcdti, especially of a specimen 

 whose relative numbers also aomewhat correspond with those of 0. siluricus. In comparing these numbers, one must 

 take into consideration that the only large shell of the last-mentioned species, which allowed measm'ement, is some- 

 what pressed in from above, so that the heights given in the table of measures may most hkely be somewhat too small. 

 The small shell, however, has a somewhat broader form than the corresponding Cambrian species, and varies in outline 

 by having the greatest breadth nearer the anterior border. 



The exterior surface of the shell of 0. siluricus has ah-eady been spoken of on several occasions: The crescent-shaped 

 growth lamelte, which correspond to a vegetative period, run out as in all Obolus shells into the posterior borders of 

 the shell, which, so to say, inclose them. The law of growth of the gi-owth lamelte, or the seam of the mantle which 

 produces the same, is plainly here and there the same. Now the abeiTancy of this species consists in the fact that the 

 concentric striae of these lamellae do not converge into the tip of the crescent as the other species — with the exception 

 of 0. antiquissimus — but run parallel to the posterior (interior) border of the lamellse and therefore always become shorter 

 toward the anterior. It is self-evident that this aberrant plaited formation must have corresponded to an aberrant 

 character of the seam of the mantle, but what the character of the latter was is unknown. Also the profile of the con- 

 centric striation of 0. siluricus deviates essentially from the other ornamented species of Eichwald's genus; for example, 

 0. triangularis shows a regular wavy line in which the crests and troughs of the wave are composed of equal curves. 

 In 0. siluricus, however, the cm'ves of the troughs follow immediately on one another, so that the crests run out into 

 sharp points. 0. antiquissimus, which has an intermediate form, shows the crests of the waves somewhat rounded off 

 at the point. 



The fringes, with which the gi'owth lamellae are ornamented at the close of the vegetative period, correspond with 

 the radial striation on the interior side of the appertaining lamellae in such a manner that every notch between two 

 teeth of fringes is equal to a radial furrow. These radial furrows, which owe their formation to the setae, which are sunk 

 into the seam of the mantle, are peculiar to all Oboli and were ah-eady observed in the impression by Eichwald in all 

 shells whose upper layer (gi-owth lamella) was broken off. To every notch of the fringe therefore corresponds a seta of 

 the mantle. The subject can not be followed any further. We also here meet with the peculiar but unknown charac- 

 ter of the seam of the mantle, which produces a pretty fringe in the vegetative rest; in the period of gi-owth, however, 

 it produces a lamella which is interrupted at the border, both being furnished with concentric ribs. The teeth of the 



