CLASSIFICATION CAMBRIAN BRACHIOPODA WALCOTT I 55 



Cardinal Area. — A more or less well-developed triangular area 

 on each side of the delthyrium, distinctly set off from the general 

 surface of the shell. It is best developed on the ventral valve of 

 articulate brachiopods, but is also present on the dorsal valve, and 

 generally in a rudimentary condition in many inarticulate species. 

 When the area is rudimentary it is often called a false or pseudo- 

 area. 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 line is called the flexure line, owing to the slight inter- 

 ruption in the striae of growth, and the spaces separated by the flex- 

 ure line are called the inner and outer lateral spaces of the area. 

 (See Deltidium and Foramen.) 



Cardinal Extremities. — The terminations of the hinge line. 



Cardinal Muscle Scar. — A large scar within which the pos- 

 terior and anterior lateral and transmedian muscle scars were at- 

 tached. 



Cardinal Process. — A variously modified apophysis, situated pos- 

 teriorly at the center of the hinge of the dorsal valve in articulate 

 brachiopoda. To it are attached the diductor muscles, which by 

 their contraction serve to open the valves anteriorly. 



Cardinal Slopes. — The inclined surfaces extending from the 

 umbonal slopes to the hinge margins. 



Central (Adductor) Muscles. — In the Protremata and Telo- 

 tremata 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 in 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 pair of centrals placed near the anterior extremity 

 of the visceral area. 



Chilidium. — A dorsal plate, in appearance similar to the del- 

 tidium, 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 there is a similar plate continuous 

 with the dorsal cardinal region of the shell, and it is named the 

 pseudochilidium. 



Crura. — Processes on the dorsal hinge plate of the Telotremata 

 and some Protremata, to which are attached the fleshy brachia and 

 brachidia. These usually form the inner walls of the dental sockets 

 and may be supported by septal plates. 



