49S 



FOSSIL BRACHIOPODA 



A " hinge area " (Fig. 334, c.a) is often present on one or both 

 valves, and may be of great size, as in Clitamhunites, but in 

 Productus it is wholly absent. In those genera that possess it a 

 triangular fissure — the " deltidial fissure " — frequently traverses 



Fk;. 021. — ('iiiir/dditnn gcdeatum. 

 Wfuloflc Liiiitbtoiie. 



Fig. -325. — Conch idium gcdeatum. 

 Transverse section. (I, Doi'- 

 sal valve ; d.s, dorsal septum ; 

 s, socket plate ; v, ventrnl 

 valve ; ns, ventral sejJtuni ; 

 d.2J, lieutal plate. 



it on l)oth valves ; in the doi'sal valve the fissure is merely the 

 space between the dental sockets, and may be occvipied by the 

 cardinal process (Fig. 334, C) or covered by a shelly plate — the 

 " chilidiuiu." In the ventral valve it gives passage to the pedicle, 



Fig. 326. — Stringocephcdus Burtini. (Modified from Woodward.) Devonian. A, Interior 

 of dorsal valve. B, Side view of interior of shell ; «, adductor ( = occlusor) scars ; 

 c, crura; c.p, cardinal process; d.s, dorsal septum; h.p, hinge plate; I, brachial 

 loop ; s/J, shelly processes ; t.s, dental sockets ; v.s, ventral septum. 



and may l)e partly or entirely closed by a similar plate (Fig. 

 334, d) known as the " pseudo-deltidium," especially large in 

 Clitamhonites, or remain open {OrtJiis). This pseudo-deltidium 

 is a primitive character, and arises in an early stage of the 



