OBOIADM. 391 



The species is distinguished from all others of the Oholus sandstone by the rounded, narrow, 

 radiating ribs, which are increased by frequent interpolation from the umbo toward the front 

 margin. The surface is also marked by fine concentric striation. Nothing is known of the 

 internal characters. 



Mickwitz states that tliis is one of the rarest of the known species of the Oholus sandstone. 



In some collections made by Schmalensee I found nearly an entire specimen of the exterior 

 of the ventral valve; this is illustrated by Plate XV, figure 3. The surface of this shell is much 

 like that of Botsfordia cselata (Hall) (PI. LIX, fig. lb). 



Formation and locality. — Upper Cambrian: (395 [ilickwitz, 1896, p. 158\) Oholus sandstone at Joa, near Jegelecht, 

 12 miles {19. S hm.) east of Reval; and (395b) Obolus sandstone at Ilgast; both in the Government of Esthonia, Russia. 



Oboltjs feistmanteli (Barrande). 



Plate XII, figures 1, la-f, 9, 9a. 



Lingula feistmanteli Barrande, 1879, Syst§me silurien du centre de la Boheme, vol. 5, PL CVI, figs, iv: 1-14; 

 PL CX, figs, viii: 1^. (No text reference. PL XII, figs. 1, la, Id, and If of this monograph are copied from 

 Barrande's figures, PL CXVI, figs, iv: 7a, 11a, 4a, and 12f, respectively.) 



General form rounded ovate to rounded subtriangular; ventral valve subacuminate, with 

 the dorsal valve rounded subacuminate; valves moderately and almost uniformly convex. 

 Surface of the shell marked by concentric undulations and striae of growth and, apparently, 

 fine radiating striae. The latter are shown only on one specimen, as the outer surface usually 

 adheres to the sandstone matrix, as in Plate XII, figure la. Wlien the outer surface of the shell 

 is exfoUated, the surface of the inner layers is marked by numerous fine radiating strisB (PI. 

 XII, figs. 1 and If), with more or less obscured concentric Unes of growth. The radiating striae 

 appear to be distributed over the entire surface of the inner lamellae of the shell, but are strongest 

 on the inner, oblique layers of the anterior portion. In some instances the striae on the oblique 

 lamellae are somewhat irregular (PL XII, fig. le); the inner surface of the shell is marked by 

 small pits or punctae and by obscure concentric lines of growth. The punct^ are distributed 

 irregularly, being most prominent in the visceral portion. The general character is well shown 

 in Plate XII, figure If. The shell is formed of a thin outer layer, with numerous inner layers 

 or lamellae over the anterior and outer portions. These inner lamellae over the anterior portion 

 of the shell are oblique to the outer layer, and when the shell is partly exfoliated they appear 

 as imbricating layers (PI. XII, figs. 1 and If). The layers or lamellae formed in the central 

 and posterior portions of the shell are relatively strong, and give considerable thickness to the 

 shell. When these are exfoliated and the obhque anterior lamellae are preserved, a shallow 

 but well-defined depressed rim extends all around the valves. This rim shows more or less dis- 

 tinctly in the cast of the interior of the shell (PI. XII, figs, lb and le). This species varies 

 somewhat in size and outline of the valves, but the variation shown is very slight in a large 

 series of specimens from the typical locaUty. A ventral valve 16 mm. in length has a width of 

 15 mm. A dorsal valve, represented in figure le, is shghtly wider than long. Usually the 

 length and width of the dorsal valves is the same. 



Casts of the interior of the ventral valve show a very clearly defined area. This is rather 

 short for so large a shell (PL XII, fig. lb) ; it is divided midway by the cast of a narrow, rather 

 deep, pedicle groove; flexure lines have not been observed. The area extends well forward on 

 the cardinal slopes and is marked by strong striae of growth paraUel to its front margin. One 

 of the casts indicates that the area formed a narrow shelf between the pedicle groove and the 

 lateral margins. The area of the dorsal valve is short and extends but a short distance out on 

 the cardinal slopes. 



Casts of the visceral cavity are well shown for both valves (PL XII, figs, lb and le). It 

 resembles that of Oholus matinalis (Hall) and 0. apollinis Eichwald. The median septum has 

 not been seen in either valve. A narrow furrow extends down the center of the dorsal valve 

 between the central scars. A similar median depression is seen in the posterior portion of the 

 visceral cavity of the ventral valve. 



The muscle scars of the ventral valve are not clearly defined. The umbonal scar is indi- 

 cated in one specimen (PL XII, fig. lb). It is divided, but no trace of a pedicle scar has been 



