438 CAMBRIAN BRACHIOPODA. 



Obolus antiquissimus Eichwald, 1859, Lethsea rossica, ancienne periode, Atlas, PI. XXXVII, figs. 5a-d. (The 



accompanying text is cited in the following reference.) 

 Obolus antiquissimus Eichwald, 1860, Lethaea rossica, ancienne p&iode, vol. 1, sec. 2, pp. 928-929. (Described and 



discussed in French.) 

 Acritis antiquissima Volborth, 1869, Verhandl. Russ.-kais. min. Gesell. St. Petersburg, 2d ser., Bd. 4, pp. 212-217, 



PL XVII, figs. 7-9. (Described and discussed in German, the description of the genus Acritis being incor- 

 porated with that of the species.) 

 Aulonotreta sculpta Kutorga, Dall, 1877, Bull. U. S. Nat. Mus. No. 8, p. 16. (Discusses the priority of the generic 



names proposed for this form.) 

 Obolus siluricus (Eichwald), Sibmiradzki, 1886, Jahrb. K.-k. geol. Beichsanstalt for 1886, Bd. .36, Hft. 4, p. 672. 



(Characterized and new locality mentioned.) 

 Acritis antiquissima (Eichwald), Gagel, 1890, Beitr. zur Naturkunde Preussens, von Physikal.-oekonom. Gesell. 



Konigsberg, 6, p. 22, PI. I, fig. 1. (Described in German.) 

 Aulonotreta antiquissima (Eichwald), Hall and Clarke, 1892, Eleventh Ann. Kept. State Geologist New York for 



1891, figs. 246 and 247, p. 244. (No text reference. Figs. 246 and 247 are copied from Volborth, 1869, PL XVII, - 



figs. 7 and 9, respectively.) 

 Aulonotreta antiquissima (Eichwald), Hall and Clarke, 1892, Forty-fifth Ann. Rept. New York State Museum for 



1891, figs. 246 and 247, p. 560. (No text reference. The figures are copied from the preceding reference.) 

 Acritis antiquissima (Eichwald), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 82, figs. 



35 and 36. (Species discussed, and priority, etc., of generic references discussed. Figs. 35 and 36 are copied 



from Volborth, 1869, PL XVII, figs. 7 and 9, respectively.) 

 Obolus (Acritis) antiquissimus (Eichwald), Mickwitz, 1896, M6m. Acad. imp. sci. St.-P^tersbourg, 8th ser., vol. 4, 



No. 2, pp. 206-213, PL III, figs. 20-22, 24-29. (Described and discussed in Gerinan; see below for translation. 



Figs. 20a-b are copied in this monograph, PL XV, fig. 2d.) 

 Obolus (Acritis) antiquissimus ventrosus Mickwitz, 1896, idem, pp. 213-214, PL III, figs. 23a-b. (Characterized in 



German as a new variety.) 



Mickwitz [1896, pp. 206-213] describes this species as follows: 



Shells large, strongly arched. Arching of the large shell slighter than that of the smaller. Greatest height of 

 both shells more toward the beak. Outline of shell broadly oval to round, broader than long, the greatest width 

 lying more to the anterior border. Beak of the large shell very blunt, laterally rounded; extreme tip of the beak of 

 the large shell lapping hook-shaped over the rectilinearly worn-off tip of the beak of the small shell. Growth lamellte 

 very thick, loosely disposed, their anterior sharp-edged borders projecting like a molding on then- inner surfaces, 

 strongly radially ribbed. Concentric striae of the outer sm-iace of the shell in wavy roundish ribs, which run pai-allel 

 to the posterior border of the crescent-shaped growth lamellse, growing constantly shorter toward the anterior border 

 of the same. Anterior and lateral borders sharp-edged, lying in one plane with the wom-off borders of the beak. Pos- 

 terior borders of the shell massive, broad, projecting inwardly in shape of a border. Area of both shells strongly devel- 

 oped, broadly ribbon-shaped, prolonged far into the lateral borders, deeply hollowed, polished on the sm-face by mutual 

 attrition. Peduncular groove strongly conical, deeply sunk, inwardly often broadened like a trumpet, traces of the 

 pseudo-area rectilinear, converging into the tip of the beak, attaining to distinctness as a line of the markation of the 

 naiTow splanchnocoelic and the broad pleurocoelic heads of the layers of the worn area lamellae. Thickening of the 

 shell extended chiefly to the brachiocoele, advancing in scalariformed terraces which follow the contom-s of the parietal 

 band; with the large shell forming a thick mass in front of the central groove, and with the small filling out the lateral 

 angulai- seams which penetrate into the splanchnocoele. 



Splanchnocoele small, crowded back strongly to the beak, reaching under the rimlike projecting posterior border 

 of the shell. Heart-shaped groove very small, very indistinctly pronounced, often only perceptible in a slightly 

 expressed furrow between the combined central places of attachment — the projecting tip of the central gi'oove. Median 

 swelling of the small shell beginning as a distinctly developed roundish ridge under the hollowed area, between the 

 places of attachment of the central muscles, which are there situated, and ending with a fine short furrow between the 

 places of attachment of the anterior lateral muscles, which are also moved very far back. Principal vessel furrows 

 of both shells deeply sunk, hollowed into plainly round or flat furrows; in the large shell more approximated to the 

 median line and running rather straight; in the small shell more approximated to the lateral borders. Secondary 

 vessels not ramified, radiating inwardly and outwardly perpendicularly to the principal vessels. 



Peduncular muscles fastened to a toothlike process, which is pushed before the opening of the peduncular groove 

 and reaches freely into the splanchnocoele. The places of attachment of the umbonal muscle, in the large shell, on 

 both sides of the place of attachment of the peduncular muscle on the nan-ow sides (perpendicular to the plane of the 

 shell border), which stands out into the interior of the shell; in the small shell on a platform, which lies opposite to 

 the place of attachment of the peduncular muscle and projects into the interior like a pier. Combined places of attach- 

 ment of the transmedian and anterior lateral muscles with the large shells in front of the divided places of attachment 

 of the umbonal muscles on the narrow side of the projecting rimlike border of the shell; with the small shell at 

 the corresponding place in similar position to the places of attachment of the transmedian muscles; and in front of 

 these the combined places of attachment of the middle and outside lateral muscles. Places of attachment of the 

 ''entral muscles in the large shell behind those of the outside lateral muscles, lying on both sides of the projecting 



