570 Forty-fifth Report on the State Museum. 



Species having the pedicle-passage situated anteriorly as in 

 S. typicalis, but having it developed as a tube, as in Siphonotksta, 

 have been referred provisionally to Schizambon {S.fissus, Kutorga, 

 S. Canadensis, Ami). These are much larger and more strongly 

 spinose shells than S. typicalis, and also occur in faunas of later 

 age. (Trenton.) 



Helmersenia, Pander. 1861. 

 (Plate 4, Figs. 6, 7.) 



Shell having the general form of Obolus, with a conspicuous 

 cardinal area depressed in the center by a broad low pedicle- 

 groove (?). Apex of the pedicle- valve truncated by a circular 

 opening, which is not produced into an internal tube. Muscular 

 impressions indistinct. 



~No specific name has been proposed for the typical form. 

 (Lower Silurian of Eussia.) 



Orbicella, D'Orbigny. 1849 (1847 '(). 

 (Plate 4, Figs. 8-10.) 



Synonym ; Keyserlingia, Pander. 1861. 



Shell inequivalve, subcircular or oval. Pedicle-valve more or 

 less conical with an excentric apex, from which diverge posteriorly 

 two low ridges inclosing a flattened triangular area 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 

 axial line. The first one-third of this tube is swollen and 

 thickened, but thenceforward it passes, with parallel sides, to 

 the anterior margins, where it is probably closed Brachial 

 valve more depressed, its greatest convexity being in the center. 

 Cardinal area as in the opposite valve, but narrower. A closed 

 tube, beginning in a swelling just in front of the cardinal line, is 

 continued from the center of the shell to the anterior margin as 

 an open channel. 



Type, Qrbicella Buch%de Verneuil (sp.). (Lower Silurian of 



Russia.) 



Discinisca, Dall. 1871. 



Synonym ; Discina, of most authors. 



Shells orbicular; brachial valve the more convex ; pedicle-valve 



more or less flattened, concave or depressed. Apices of both 



.1 L>1> 



