336 



POPULAR CONCHOLOGY. 



Shell transverse, equilateral, hinge linear, straight, widely 

 extended on both sides of the 

 umbones, which are separated by 

 a flat area in the upper or larger 

 valve ; this area is divided in the 

 centre by a triangular pit for the 

 passage of the byssus ; interior 

 with two spirally convolute appen- spirifer trigonous. 



dages.* — Fossil. 



Distinguished from Terebratula by the flat area in one 

 valve, and the singular spiral processes, from which the 

 name is derived. Dr. Mantell observes that the calcareous 

 convoluted appendages in the interior of this shell, are 

 often beautifully preserved in the fossil state ; they seem 

 to have been as supports, to the ciliated arms of the 

 animal when living. 



Cyrtia. Dalman. (Acrotreta Kutorga.) — Shell with 

 hinge rectilinear; the back elevated into a semi cone, or 

 half pyramid, the cardinal side perpendicularly plane.f — 

 Fossil. 



Atrypa. Dalman. Actinoconchus, M'Coy.) — Shell 

 nearly equivalve, equilateral, both valves convex ; hinge 

 furnished with two spiral appendages ; its margin short, 

 arched ; the area between the beaks small, generally 

 convex, rarely flat, with a triangular pit or notch in the 

 middle, closed by a deltidium admitting the smaller beak ; 

 beaks pointed, incurved.J — Fossil. 



Athyris. M'Coy. ( Spirigera U Orb. — Shell nearly 

 orbicular, small ; no cardinal area, or hinge line ; spiral 

 appendages very large, filling the greater part of the shell. 

 — Fossil. 



This genus is established for such Terebratula as combine 



* Sovverby's Mineral Conch. 



t lb. + lb. 



