498 



FOSSIL BRACHIOPODA 



CHAP. 



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

 both valves, and may be of great size, as in Clitamhonites^ but in 

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

 triangular fissure — the "deltidial fissure" — frequently traverses 



Fig. 324. — C'onchidUini yalea- 

 tum. W^enlock Limestone. 



Fig. 325. — Conchidium galea- 

 tum. Transverse section. 

 d, Dorsal valve; d.s, dorsal 

 septum; s, socket plate; v, 

 ventral valve; v.s, ventral 

 septum ; d.p, dental plate. 



it on both valves ; in the dorsal valve the fissure is merely the 

 space between the dental sockets, and may be occupied by the car 

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

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



Fig. 326. — Stringocephahis Burtini. (Modified from Woodward.) Devonian. A^ 

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

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

 I, brachial loop; s.p, shelly processes; i.s, dental sockets; v.s, ventral septum. 



and may be partly or entirely closed by a similar plate (Fig. 

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

 Clitambonites, or remain open (Orthis^. This pseudo-deltidium 

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



