TEEMINOLOGY. 293 



valve of articulate brachiopods, but is also present on the dorsal valve, anci generally in a 

 rudimentary condition in many inarticulate species. Wlien the area is rudimentary it ia often 

 called a false or pseudo-area (PI. II, figs. 1 and 7a; PI. Ill, figs. 2a and 6b; PI. V, figs. Id at a; 

 PI. VI, figs. 1" and 3b). The area of some of the inarticulate genera is frequently divided by 

 a line between the delthyrium and the outer margin; in such areas the Ime is called the flexure 

 line, owing to the sUght interruption in the strise of growth (PI. VIII, fig. Id at f), and the 

 spaces separated by the flexure line are called the inner and outer lateral spaces of the area. 

 (See Deltidium and Foramen, and p. 303.) 



Cardinal extremities. — The terminations of the hinge line. 



Cardinal muscle scar. — A large scar within wliich the posterior, anterior lateral, and 

 transmedian muscles were attached. 



Cardinal process. — A variously modified apophysis situated posteriorly at the center of 

 the hinge of the dorsal valve in articulate brachiopods. To it are attached the diductor 

 muscles, wliich by their contraction serve to open the valves anteriorly. (See p. 306.) 



Cardinal slopes. — The inclined surfaces extending from the umbonal slopes to the hinge 

 margins. 



Central {adductor) muscles. — In the Protremata and Telotremata these muscles have 

 their ventral insertion one on either side of the central axis, between the diductors. In 

 passing to the dorsal valve they divide into four, and produce m that shell the two pairs of 

 principal scars known as the anterior and posterior centrals. By contraction these muscles 

 close the shell. In the Neotremata they are the essential muscles, the anterior centrals closing 

 the valves, while the posterior pair serves to open the valves. In the Atremata there is a 

 simple j)air of centrals placed near the anterior extremity of the visceral area. 



Ghilidium. — A dorsal plate, in appearance similar to the deltidium, covering the exterior 

 portion of the cardinal process in many Protremata. Its development does not begin until 

 early neanic or later growth, and it is probably secreted by the dorsal mantle lobe. In the 

 Atremata and Neotremata a similar plate, continuous with the dorsal cardinal region of the 

 shell, is named the pseudochilidium. (See p. 306.) 



Crura. — Processes on the dorsal hinge plate of the Telotremata and some Protremata, 

 to wliich are attached the fleshy brachia and bracliidia. These usually form the inner walls 

 of the dental sockets, and may be supported by septal plates. 



Cruralium. — The dorsal equivalent of the ventral spondylium. (See p. 309.) 



Deltliyrium.. — The triangular aperture transecting medially the ventral cardinal area, or 

 the posterior surface from the apex to the posterior margin of the ventral valve, through some 

 portion of which the pedicle passes. It has also been termed the fissure or foramen. The 

 delthyrium may or may not be closed either by a calcareous deltidium or a phosphatic pseudo- 

 deltidium. (See p. 305.) 



Deltidium. — A plate more or less continuous with the cardinal margm of the ventral 

 ■valve and covering the delthyrium in Atremata, Neotremata, and Protremata. Wlien present in 

 inarticulate brachiopods it is called the pseudodeltidium. In the Protremata, in which it is 

 always more calcareous, thicker, and more sharply defined, it is called the deltidium or the 

 pseudochilidium. (See p. 305.) 



Dental plates. — Vertical plates supporting the teeth of the ventral valve in articulate 

 brachiopods. (See p. 310.) 



Dental sockets. — Excavations in the dorsal cardinal margin of articulate brachiopods, in 

 which the teeth of the ventral valve articulate. The inner wall of the socket is elevated and 

 forms the base of the crural plate. 



Diductor muscles. — In the Protremata and Telotremata the pi-incipal pair of diductor 

 muscles has the larger end attached to the ventral valve near the anterior edge of the visceral 

 area, and the other end has its insertion on the anterior portion of the cardinal process. By 

 contraction these muscles open the valves. 



Dorsal valve. — Usually the smaller and imperforate valve and the one to which the brachia 

 are always attached. Brachial, Tisemal, socket, and entering valves are other terms more rarely 

 employed. 



