DEFINITIONS OF BRACHIOPOD TERMS 7 



Chilidium (pi. A, figs. 4, 6, 7). — ^The covering of the notothyrium, the dorsal equivalent of 

 the ventral deltidium. 



Commissure. — The boundary line between the anterior and lateral margins of the valves. It 

 may lie in a single plane or be flexed to a greater or less degree. See Plane of commissure and 

 Rectimarginate. 



Convexity. — In describing the convexity of a brachiopod, the dorsal valve is named first; this 

 is for the sake of making comparisons always in the same direction, namely, from dorsal above to 

 ventral below. For example, Valcourea defecta would be spoken of as convexo-concave and Orthis 

 callactis would be termed concavo-convex. See Resupinate. 



CosTA (cost/-e). — A coarse radial rib on the external surface. When a species has costx that 

 are simple and distant it may be called Paucicostate; when the costas are numerous and increase by 

 bifurcation or implantation it is Multicostate; and when they are bundled into fascicles the term 

 Fascicostate may be employed. Costas may be angular, subangular, or rounded. 



Costella (costell^). — Fine external ribs such as those of Dalmanella. The same prefixes 

 may be used in connection with this term as with costje. Extremely fine radial lines, such as those 

 on the costs of Orthis s. s., may be termed Parvicostell^. 



Crura (sing. Crus). — Processes in the dorsal valve of the Telotremata to which are attached 

 the fleshy brachia or the brachidia. See Brachiophore. 



Crural fossette (pi. A, fig. 15). — An excavation on the inner face of the tooth of the ventral 

 valve which receives the postero-ventral edge of the dorsal brachiophore in the articulation of the 

 shell. 



Crural plate. — A general term commonly applied to the brachial processes of the orthids, 

 strophomenids, and rhynchonellids, without regard to detailed structure, function, or homologies. 



Cruralium (pi. 18, fig. 17; pi. 25, fig. 24). — The dorsal equivalent of the ventral spondylium 

 {q. v.). When the brachial lamellse unite with a median septum, the whole structure is called a 

 Cruralium simplex; when the brachial plates remain discrete, do not have the muscle attachments, 

 and do not unite with a median septum (as in Porambonites), the structure is known as a Discrete 

 CRURALIUM; and when there is no median septum, it is called a Sessile cruralium. 



Deltarium. — See Deltidial plates. 



Delthyrial cavity (pi. A, fig. 15). — The ventral umbonal cavity bounded by the dental 

 plates. 



Delthyrium. — The triangular aperture which transects the ventral interarea medially, and 

 through some portion of which the pedicle passes. It has also been termed the Fissure. The del- 

 thyrium may or may not be closed by a deltidium or deltidial plates. Its equivalent in the dorsal 

 valve is the notothyrium {q. v.). 



Deltidial plates. — In Telotremata, two plates growing medially from the walls of the del- 

 thyrium after neanic growth. These often unite medially, closing the delthyrium more or less com- 

 pletely. When united, they make a Deltarium = symphytium of Buckman, pseudodeltidium of 

 Schuchert (not Bronn or Walcott). In Protremata, similar plates are at times developed and these 

 are called Lateral plates (q.v.). 



Deltidium (pi. A, fig. 2). — An independent, more or less strongly arched plate in the ventral 

 palintrope or cardinal area in many Protremata, growing from the apex toward the hinge-line and 

 partly or completely covering the delthyrium. It is always delimited from the mterarea by grooves. 

 It is characteristic of primitive shells, and is formed by a flap of the ventral mantle. 



Dental plates or Dental lamell.^ (pi. A, fig. 5).— Vertical or nearly vertical plates associ- 

 ated with the teeth of the ventral valve, usually uniting the palintrope to the floor of the valve, and 

 bounding the delthyrial cavity. They are separated from the walls of the shell by the umbonal cavi- 

 ties When the latter have been filled by adventitious shell, the dental plates become Obsolete. A 

 spedal type of dental plates seen in Hesferorthis, etc., is called Receding dental plates. These 

 are ridges along the ventral surface of the palintrope, which finally reach the mner surface of the 

 valve at the posterior of the shell only. 



