436 CAMBRIAN BRACHIOPODA. 



fringes, at the anterior border of the adult shell, reach a considerable length (four to five transverse ribs): near the beak 

 they are only indicated. 



The breadth of the growth lamella of this species varies gi-eatly.. One shows in the center of the shell a width of 

 5.5 mm., while the center lamella of another is only 2 mm. broad. This is plainly connected with the relative condi- 

 tions of nourishment. Independent thereof also here, as with all Obohis species, a diminishing of the breadth of the 

 lamellse toward the anterior border of the adult shell takes place. 



The shells of 0. siluriciis are very thin, and consequently the development of the inner characters is very slightly 

 pronounced. In striking contrast to this peculiarity stands the strongly developed area with the broad, deeply hollowed 

 peduncular groove, which somewhat diminishes toward the beak. 



The surface of the area is smooth at the posterior part, and gives rise to the conjecture that in consequence of a 

 narrow closing of the shell a mutual attrition has taken place. (Similarly as in 0. antiquissimus, only in a less degi'ee.) 

 The same conclusion is forced upon one by the deeply hollowed peduncular gi-oove, which allowed, even with the 

 narrow closing of the shell, sufficient space for the passing of the peduncle; and lastly, the strongly developed longitudi- 

 nally striate places of attachment of the umbonal rnuscle. A small shell of a young specimen also shows this place of 

 attachment distinctly but lacks the striation in the figure. 



The beak of the small shell also has in its youth a rectilinear bordered tip. In age this is somewhat worn oft, so 

 that the beak has a round appearance. The area of the small shell is hollowed transversely to the median line and 

 therefore not exposed to attrition, which is limited to the borders of this shell. Unfortunately this part is only at hand 

 in the already mentioned young specimen. 



The inner characters of 0. siluricus are more withdrawn from observation. On the one hand they are, as already 

 mentioned, very slightly developed; on the other hand sufficient material is lacking. The central gi'oove of the large 

 shell is very flat and indefinitely bordered, but is perceptible in both figured specimens. 



The principal vascular fm-rows are more distinctly distinguishable, their central swellings are strongly developed 

 and rear themselves from the deeply hollowed lateral fm'rows. In one specimen the secondary vascular traces may be 

 followed under a magnifying glass. The accessory vessels, radiating inwardly and perpendicularly to the principal 

 vessels, run rectilinearly to the anterior border, and bend off shortly before reaching the median line, as has been 

 demonstrated in other species. A shortly branched ramification of the first part of these vessels was only suggestively 

 perceptible. The outwardly directed accessory vessels run in the known manner. Their opening into the peripheral 

 canal is not perceptible on account of corrosion of this part of the shell. 



Of the remaining visible characters in the figm'ed large shell only the places of attachment of the combined central 

 muscles are to be mentioned, which, however, can not be analyzed into their elements on account of the deficient pre- 

 servation of the shell. In their totaUty, they form a narrow band, which lies before the central groove and is bordered 

 by the parietal band. This band curves, close to the inner lateral furrows of the principal vessels, rectangularly toward 

 the back and is widened there considerably. This portion, however, is corroded, so that the exact form of the place of 

 attachment can not be ascertained. 



In the small shell, apart from the already mentioned place of attachment of the umbonal muscle, the large chcular 

 places of attachment of the central muscles may be seen for the first time, and lie near the center of the fragment of the 

 shell on both sides of the flat median swelUng, which is divided by a broad median furrow. The places of attachment 

 of the anterior lateral muscles are also perceptible, but only suggestively. They already lie in the scaled-off part of 

 the anterior half of the shell. On the left above, the elongate, quite deeply sunk place of attachment of the transmedian 

 muscle is also perceptible. Lastly, the beginnings of the principal vascular traces and parietal band, as far as the 

 inner surface of the shell is intact, may be followed and are also expressed in the figure. 



The species owes its specific name to the "Silurian" age of the beds in which it is found. 



Formation and locality .« — Passage beds between the Upper Cambrian and the Ordovician: "Glauconite 

 sandstone" at the following localities: (396) At Baltischport, 30 miles (48 km.) west of Reval; (396a) at LeppiJco, near 

 Leetz, on the eastern side of the Baltischport Peninsula, about 25 miles {40.3 Tan.) west of Reval; (396b) at Fall, 15 miles 

 (24 km.) west of Reval; (396c) at the mouth of Fahna Brook, east of Fall, about 15 miles (24 km.) west of Reval; and 

 (396e) in the eastern part of the east Baltic region; all [Mickwitz, 1896, p. 198] in the Government of Esthonia, Russia. 



(395x [Lamansky, 1901, p. 617]) Glauconitic sandstone (correlated by Lamansky mth the Upper Cambrian Cera^ 

 topyge slate) in the vicinity of Baltischport, 30 miles (48 km.) west of Reval; (395v [Eichwald, 1843, p. 147]) sandstone 

 on Odensholm Island, about 55 miles (88.6 km.) west of Reval; (396y [Eichwald, 1843, p. 147]) chlorite-bearing sands 

 in the limestone beds at Reval; (336f [Eichwald, 1860, p. 927]) pyi-oxene-bearing sands in tlie Orthoceratite limestone 

 beds at Reval; (337i [Schmidt, 1861, p. 218]) green sand at Baltischport; and (396x [Eichwald, 1843, p. 146]) chlorite- 

 bearing sands in the limestone beds at Baltischport, about 30 miles (48 km.) west of Reval; all in the Government of 

 Esthonia, Russia. 



(395w [Eichwald, 1843, p. 147]) Chlorite-bearing sands in the limestone beds at Pawlowsk; and (395y [Kutorga, 1848, 

 p. 281]) sandstone on Pulkowa Brook; both near St. Petersburg, Russia. 



(368a [Siemiradzki, 1886, p. 672]) Gray sandstone of unknown stratigi'aphic position or age, in the Polnischen 

 Mittelgebu'ge, north of Sandomierz on the Vistula, Russian Poland. 



a Localities 396 and 396a are represented in the collections ol the United States National Museum. 



