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 liave 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. 



ISCil. Keyseiiingia, Pander. Bull, de I'Acad. Imp. des Sciences de St. Petersbourg, vol. iii, p. 46, pi. 

 ii, fiif. la-h ; in Helinei-sen'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. Fi-om this apex diverge posterioi'ly 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 Helmersenia and Obolus. In the 

 interior of the valve a closed tube is continued from the external fissure, in the 



