572 Forty-fifth Report on the State Museum. 



Orbiculoidea, D'Orbigny. 1847. 

 (Plate 5, Figs. 11-17.) 



Synonym ; Discina of authors. Not Orbiculoidea, Davidson. 

 1856. 



Shells subcircular or subelliptical in outline, inequivalve. 

 Apices excentric. Pedicle-valve depressed-convex or flattened, 

 with the apex slightly elevated and inclined posteriorly. On the 

 exterior of the valve a narrow pedicle-furrow, abruptly inter- 

 cepting the ornamentation but not penetrating the substance of 

 the shell, begins just below and behind the apex, extends over a 

 greater or less portion of the radius of the valve, and, at its 

 distal end, is produced into a short tubular sipho, which traverses 

 the substance of the shell obliquely backward, emerging on the 

 interior surface, where it produces a narrow groove, and usually 



terminates before reaching the 

 margin of the valve. On the 

 interior, the position of the ex- 

 ternal groove is marked by a 

 fig. 265 - vertical section of the pedicle- valve thickened ridge extending f rom 



of Orbicidoidea. ^ ^^ and tnig fc CO ntinUOUS 



with the thickened margins of the internal groove, which, in 

 advanced age, may become so developed as to envelop this groove, 

 except at its outer end. 



The larger or brachial valve is depressed-conical, with the apex 

 more strongly directed backward than in the opposite valve. 

 The interior shows a fine longitudinal ridge or septum extending 

 from the apex forward. Otherwise the internal markings are not 

 satisfactorily known. 



Shell substance composed of alternating lamella? of corneous 

 and mineral matter, the latter often removed in fossilization, 

 making the shell appear essentially phosphatic. Surface orna- 

 mentation usually consisting of fine, crowded or distant, some- 

 times lamellose, concentric lines, occasionally crossed by radiating 

 lines or ridges. 



Type, Orbicula Mbrrisi, Davidson. I Wenloek limestone.) 



Distribution. Lower Silurian — Cretaceous (?). 



124 



