OBOLID^. 543 



broader, and the form of the outline approaches more to the elliptic. At the same time the shell is much more strongly 

 arched. A small shell -n-hose outline deviates only slightly from the typical form is arched much more than the nor- 

 mal specimens. These transition forms point, as we see, toward the variety solidus. 



The typical form shows two slightly pronounced roundish edges which converge into the tip of the beak and 

 are caused by a lateral flattening of the posterior part of the shell. In like manner the anterior part of the shell is 

 somewhat flattened towaixl the anterior border. These chai-acters are first plainly perceptible by reflected light, 

 and are much less developed in the described roundish foi'ms. A peculiar coiTelation also seems to exist between 

 the form of the shell and the strength of the shell. The latter is the slightest in the typical form, increases in the 

 roundish form, and acquh-es the maximum in 0. lingul^formis solidus. 



The thinness of the shell of the species in question is pronounced, especially in the deficient thickening of the 

 tip of the beak, which in this regard reminds one of the recent lingulas. Here, as well as there, the broad posteriorly 

 diminished peduncular groove is shallow and only indefinitely bordered, and sunk in the area of the large shell. 0. lin- 

 gulceformis reminds one, more than all other species of Eichwald's genus, in the foi-mation of the area of the small 

 shell, of the recent Lingula with which this pai't is, to be sure, in a still more pronounced manner, a mere covering 

 of the thin- walled tip of the beak with the corneous area lamellae. 



The configuration of the inner surface of the shell is, in relation to the deficient thickening, quite distinctly devel- 

 oped. In its projecting tip the splanchnocoele of the large shell in some measm-e resembles the corresponding formation 

 of Lingula anaiina. On the other hand, the small shell shows the three-lobed form, which is peculiar to the genus 

 Obolus and is continued by the aberrant position of the places of attachment of the muscles. 



The thickening of the posterior part of the large shell is reduced to a wall-like rim whose pointed upper edge 

 incloses the parietal band. The declivity of this rim into the brachioccele is quite steep and marked at its middle, 

 circular-formed, projecting part by somewhat diverging longitudinal swellings palmately arranged ; the slope into the 

 splanchnocoele is flatter and is lost in the indefinitely bordered central groove, from which only the strongly drawn 

 forth, parallel-bordered, somewhat anteriorly broadened, and rounded-off tip is clearly distinguishable by a slight 

 but sharply bordered depression. 



As often mentioned, the places of attachment of the middle lateral muscles, which are removed from the places 

 of attachment of the central muscles, and united to one surface, lie before the tip of the central groove. The places 

 of attachment of the central muscles are oval and lie on both sides of the drawn-out apex of the central groove. They 

 show a coarse transverse striation, which is posteriorly continued farther than the places of attachment, and owes its 

 origin to the gradual advance of the central muscles, whose traces were only partly covered by a slight secretion of 

 lime. A similar condition takes place with the places of attachment of the outside lateral muscles of the large shell, 

 which lie, as with Schmidtia, close to the inner sides of the principal vascular canal, far removed from those of the cen- 

 tral muscles, and somewhat pushed back. Yet more striking is this succession of exposed traces with the places of 

 attachment of the central muscles of the small shell, in which it may be followed from the inner sides of the corneous 

 processes to the extreme tip of the sinus. 



The last-mentioned small shell of the variety solidus shows very distinctly — namely, at the right side — the places 

 of attachment of the combined middle and outside lateral muscles as well as those of the transmedian muscles, while 

 the umbonal muscle has left less distinct traces. The first-mentioned places of attachment lie so close to the border 

 of the shell that the pleurocoeles are reduced to small, ribbon-shaped strise. This form of the pleurocoele stands 

 in connection with the previously mentioned law of growth of the shells. 



In the large shell the places of attachment of the combined anterior lateral and transmedian muscles, likewise 

 on the right side, are quite distinctly pronounced, while the places of attachment of the divided umbonal muscles 

 and peduncular muscle form a somewhat indefinitely bordered, coherent surface, which is longitudinally striate in 

 the middle. 



Very striking to the eye is the strongly developed median swelling of the small shell, which separates the dis- 

 tinctly depressed oval places of attachment of the anterior lateral muscles, and reaches, with its pestle-shaped swelled 

 ends, nearly to the anterior border of the shell; of the circulatory system only the posterior parts of the principal 

 vessels (with the exit out of the splanchnocoele) are unfailingly distinguishable. The concentric strise of the small 

 shell are insufficiently covered furrows of separation of the growth lamellas. 



Formation and locality. 'i' — Passage beds between the Upper Cambrian and the Ordovician: "Glauconite sand- 

 stone" at the following localities: (396) at Baltischport, 30 miles (48 km.) west of Reval; (396a) at Leppiko near Leetz, 

 on the eastern side of the Baltischport peninsula, about 25 miles (40.3 km.) 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; (396d) at 

 Domglint in Reval; and (396f) in the western part of the east Baltic region; all [Mickwitz, 1896, p. 203] in the Gov- 

 ernment of Esthonia, Russia. 



Upper Cambrian: (395) Obolus sandstone at Joa, near Jegelecht, 12 miles (19.3 km.) east of Reval, Government 

 of Esthonia, Russia. 



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



