74 SYNOPSIS OF AMERICAN FOSSIL BRACIIIOPODA, [bull. 87. 



J^cJihi/rium (Hall and Clarke). — The triangular aperture transecting 

 medially the cardinal area, or the posterior surface from the apex to the 

 l)osterior 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 by a deltidium or deltidial 

 plates. 



Deltidium. — A plate of one piece which grows over the delthyrium 

 of many Protremata and some ^Neotremata. In the early larval stage 

 of Thecidium this idate begins as a secretion from the dorsal side of 

 the body segment, and becomes anchylosed to the ventral valve in the 

 phylembronic stage, subsequent additions being secreted by the body 

 wall and j)edicle. The convex or concave central portion of the ventral 

 cardinal area in some Atremata is not homologous with the deltidium. 

 It is but a part of the area, and does not have its origin in the prodel- 

 tidium, as in Thecidium. 



Deltidial plates. — Two plates growing medially from the walls of the 

 delthyrium after neanic growth. These usually unite medially, and 

 close the delthyrium more or less completely. They are restricted to 

 the Telotremata, and are secreted by extensions of the ventral mantle 

 lobe. Hall and Clarke introduced the terms deltarium and deltaria for 

 the same plates, and for the coalesced condition of the deltaria, Bronn's 

 l^seudodeUidium. 



Dental plates. — Vertical plates supporting the teeth of the ventral 

 valve. 



Dental soelcets. — Excavations in the dorsal cardinal margin 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. 



Diduetor muscles. — In the Protremata and Telotremata the principal 

 pair of diduetor muscles 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 anterior portion of the cardinal process. There 

 is another pair of small accessory diduetor muscles, but these are sel- 

 dom shown in fossil shells. By contraction these muscles open the 

 valves. 



Dorsal valve. — •Usually the smaller and imj^erforate valve and the 

 one to which the brachia arc always attached. Brachial, licemal, socket, 

 and entering valves are other terms more rarely employed. 



Dphehie (Hyatt, emend. Bather and Buckman). — Designating the 

 mature shell. 



Foramen. — A small circular passage through the deltidium or del- 

 tidial plates, either below or at the apex of the ventral valve. Some- 

 times the foramen encroaches by abrasion upon the umbo of the ventral 

 valve. 



Genital markings. — Radial markings or pits within the posterior 

 portion of the visceral space, indicating the position and extent of the 

 genitalia. 



