846 Report or tHE State GEOLOGIST. 
Pedicle-valve with well-developed median septum, and moderately 
long spondylium which is free for fully two-thirds its length. 
In the brachial valve of typical examples from the Gotland and 
Wenlock limestones no spondylium is formed; the septa support- 
ing the crural plates resting directly on the surface of the valves. 
Fie. 459. Fie. 460. 
Fig. 461.—'Transverse section of Sieberella galeata, near the beaks, the pedicle-valve being 
uppermost; showing the discrete septa of the brachial valve. 
Fig. 462 — Sieberella Sieberi, von Buch. Transverse section, showing the form of the 
spondylia, 
This is, however, a variable feature, the septa not infrequently 
forming a spondylium supported by a median septum. 
Type, Sieberella Siebert, von Buch (sp.). 
Distribution. Upper Silurian — Lower Devonian. 
Gypidula, Hall. 1867. 
(Plate 48, figs. 22-28.) 
Shells with the contour of Sreperetxa, plicate or smooth. 
Pedicle-valve with a well-defined, true, cross-striated cardinal 
area, and narrow, but erect or convex, in- 
cipient deltidial plates. On the interior the 
teeth are unusually strong, the septum very 
short, the spondylium being free for most of 
its length. In the opposite valve the dental 
Fie. 461.—Gypidula sockets are distinct, the crural: plates ex- 
crarigento » Seetit panded nearly horizontally, being divided at 
pre ee ie oe their beginning by a narrow median cardinal 
valve; showing theform process. The inner moiety of the crural plates 
Of the setae is deflected to a vertical or slightly divergent 
position and in this form they are produced anteriorly. These 
plates rest upon two broadly convergent septa which unite with 
the valve making a sessile spondylium, which is acute at its 
anterior extremity, and lies at, or in front of the center of the 
98 
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