156 SMITHSONIAN MISCELIvANEOUS COLLECTIONS VOL. 53 



Cruralium. — The dorsal equivalent of the ventral spondylium. 



DeIvThyrium. — 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 pseudodeltidium. 



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

 margin on the ventral valve covering the delthyrium in Atremata, 

 Neotremata, and Protremata. When present in inarticulate brach- 

 iopods it is called the pseudodeltidium, and in the Protremata, where 

 it is always more calcareous, thicker, and more sharply defined, the 

 deltidium and pseudochilidium. 



Dental Plates. — Vertical plates supporting the teeth of the ven- 

 tral valve in articulate brachiopods. 



Dental Sockets.— Excavations in the dorsal cardinal margin of 

 articulate brachiopods in which the teeth of the ventral valve articu- 

 late. The inner wall of the socket is elevated and forms the base of 

 the crural plate. 



DiDucTOR Muscles. — In the Protremata and Telotremata the 

 principal pair of diductor 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. 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, hcemal, 

 socket, and entering valves are other terms more rarely employed. 



Ephebic. — Designating the mature shell. 



False Area. — See Cardinal area. 



Flexure Line. — See Cardinal area. 



Foramen. — A small circular passage through the deltidium, either 

 below or at the apex of the ventral valve. Sometimes the foramen 

 encroaches by pedicle abrasion upon the umbo of the ventral valve. 



Foraminal Tube. — The pedicle opening through the ventral 

 valve of Neotrematous genera. 



Genital Markings. — Radial markings or pits within the pos- 

 terior portion of the visceral space, indicating the position and ex- 

 tent of the genitals. 



Gerontic. — Designating old age. It is indicated in the ontogeny 

 of many species of brachiopods by extreme thickness of the valves, 

 obesity, or by numerous, crowded growth lines near the anterior 

 margin — a condition which sometimes produces truncation and ab- 

 sence of striae at the margin. 



