g GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Dental sockets. — Excavations in the dorsal cardinal margin in which the teeth of the ventral 

 valve articulate. The inner wall of the socket is bounded by the brachiophore in orthids. Acces- 

 sory DENTAL SOCKETS, SCe SoCKETS. 



Denticles (pi. 10, fig. 21). — Small processes on the posterior surface of the dorsal socket 

 which are inserted into the accessory sockets in the ventral teeth. 



DiDucTOR MUSCLES (pi. A, fig. 13). — The muscles that open the valves. In the Protremata 

 and Telotremata the principal pair has the larger end attached to the ventral valve near the anterior 

 edge of the visceral area, while the other end has its insertion on the posterior portion of the cardinal 

 process. There is another pair of small accessory diductor muscles, but scars of these are seldom 

 shown in fossil shells. When present they are situated just behind the adductor impressions. 



Dorsal direction. — Toward the dorsal valve, at right angles to the plane of the commissure. 



Dorsal median ridge (pi. A, figs. 7, 8). — A low axial thickening on the dorsal interior of 

 most orthids and rhynchonellids. See Median septum. 



Dorsal valve. — Usually the smaller valve, and the one to which the brachia are always 

 attached. Brachial and Socket valve are other terms less often used. 



In this work we shall stand by the biological usage of naming the two valves ventral and dorsal, 

 a position that is fortified by the work of Conklin,* which showed, from the orientation of the embryo, 

 that the valves are truly ventral and dorsal, although in nature the adult shells usually have the 

 ventral valve uppermost. 



Duplex spondylium. — See Spondylium simplex. 



Endopunct/E. — See Punct;e. 



Euseptum (pi. A, fig. 5). — ^A ventral median ridge formed by the inner shell layers rising 

 into a septum that usually makes the seat of attachment for the adductor muscles. 



ExopuNCT^E. — See Punct^. 



Fascicostate. — See Cost^. 



Fascicostellate. — See Costell^. 



Fil;e. — Fine elevated concentric lines. 



Fold (pi. 24, figs. 5, 19). — A broad median external undulation or plica that may be situated 

 on either the dorsal or ventral valve, hence dorsal or ventral fold. It is more commonly on the 

 dorsal valve. Its counterpart is Sulcus, q. v. 



Foramen. — See Pedicle foramen. 



FossETTE. — See Crural fossette. 



FuLCRAL PLATES (pi. A, fig. 14). — Small concave plates attached to the outside wall of the 

 brachiophore support or brachiophore and the inner wall of the shell. These serve to define the 

 sockets and strengthen the brachiophore supports. 



Genital markings (pi. 12, figs. 20, 24). — Radial markings, ridges, or pits within the 

 posterior portion of the visceral space, indicating the position and extent of the genitalia, and used 

 for the attachment of muscles which fix the ovarian bodies. Best seen on the ventral shell. 



Gerontic. — Signifying old age. 



Hinge-line. — ^The line along which articulation takes place. 



Hypercline. — See Pt. Ill, Interareas, and t. fig. 1. 



Interarea. — The posterior plane or curved surface lying between the apex and the line of 

 valve junction. Formerly called cardinal area. 



Lateral areas. — The parts of the valves on either side of the median axis or on either side of 

 the fold and sulcus. 



^ Conldin, E. G., The embryology of a brachiopod, Terebratulma seftentrionalis Couthouy. Proc. Amer. Philos. Soc., 

 vol. 41, 1902, pp. 41-76. 



