BRACHIOPODA. 
117 
species have been kindly placed at our disposal by Mr. Walcott, and the 
representations of the interior of both valves given upon Plate IV show the 
muscular impressions as clearly as we have been able to make them out, and 
are in essential agreement with Mr. Walcott’s conception of them. The in¬ 
terior of the pedicle-valve shows a broad, low sinus with its slightly elevated 
margins diverging outward from the foramen. Two lateral diverging ridges 
also take their origin near the beak, while the track of the pedicle-aperture is 
marked on this surface by a low axial incision. In the opposite valve is a low 
median ridge with two or more pairs of diverging lateral ridges, the outer 
of which is the stronger. The extremities of the intermediate ridges fre¬ 
quently show a subcircular discoloration, which may indicate the last implanta¬ 
tion of the lateral muscular bands. The whole muscular area appears somewhat 
thickened and elevated. In S. ? Canadensis, we have found only evidence of a 
strong median septum. 
Schizambon typicalis appears to be the earliest representative of this group, 
being from the lower part of the “ Pogonip group ” of Mr. Walcott, or the 
lowest Silurian (Primordial ?) of the Eureka District, Nevada. It is therefore 
the predecessor of the other members of the genus, which are from the later 
faunas of the Lower Silurian; a fact which may account for the divergence of 
the latter, in some respects, from the type-species. 
Genus KEYSERLINGIA, Pander. 1861. 
PLATE IV, FIGS. 1-3. 
1861. Keyserlingia, Pander. Bull, de l’Acad. Imp. des Sciences de St. Petersbourg, vol. iii, p. 46, pi. 
ii r fig. 1 a-h; in Helmersen’s “Die geolog. Beschaffenh. d. unt. Narovathals,” etc. 
Diagnosis. “ Shell inequivalve, subcircular or oval. Ventral valve more or 
less conical, with a prominent apex which is sometimes near, sometimes distant 
from the cardinal margin. From this apex diverge posteriorly two low ridges 
enclosing a flattened triangular area, which is divided in the center by a vertical 
fissure. The lines of growth cross the hinge-line, forming in the center a 
faintly depressed triangular groove, as in IIelmersenia and Obolus. In the 
interior of the valve a closed tube is continued from the external fissure, in the 
