468 Forty-fifth Report on the State Museum. 



beginning (a, a), the umbo (u), the more prominent portion or 

 beak including the apex; the umhonal slopes are the inclined 

 surfaces about the umbo, except those extending to the hinge 

 margins, which are known as the cardinal slopes (c). The 

 hinge-line (h) is the straight or curved line along which articula- 

 tion takes place; the cardinal extremities (e) the exsert termina- 

 tions of this line ; the cardinal area (c, ca), a triangular vertical or 

 curved surface, distinctly set off from the general surface of the 

 shell and more highly developed on the pedicle-valve. This 

 cardinal area is transsected by a median triangular aperture 

 having its apex at the apex of the pedicle-valve, its base at the 

 hinge-line ; this is called the delthyrium (dt) and in function it is a 

 passage for the pedicle. 



This aperture is usually closed or covered at some period in 

 the history of the organism ; in the earliest growth-stages of the 

 shell, and in primitive genera, it is covered by a single convex 





■A-y"' 



A 



Fig. 12.— Spirifer. 



plate or sheath, known as the deltidium (d). In secondary or 

 derived genera (Rhyncboniilla, Spirifer, Athykis, Terebratui a) 

 the deltidium disappears by resorption, at a very early stage of 

 growth, or is never present, and the delthyrium may remain open 

 for a considerable period in the life of the animal; but it eventu- 

 ally becomes more or less completely closed by the gradual 

 formation, along the lateral margins of the delthyrium, of two 

 separate plates (delttidial plates, dp), which in the adult condition 

 may either remain discrete or unite along the median line, or 

 haying thus united, become coalesced intoasingle plate (Spirifer, 

 Cirtina), which has the form of a deltidium but is wholly dis- 

 tinct in origin. Both deltidium and deltidial plates may enclose 

 an oval or circular passage or foramen (f) i'ov the protrusion of 



20 



