CHAPTER II. 



BRACHIOPOD TERMIX0L0(;Y APPIilF.D TO FOSSIL FORMS. 



Adductor muscles. — In tbe Protremata and Telotremata these muscles 

 have their ventral iusertion one on either side of the central axis, 

 between the diductors. In i)assing- to tlie dorsal valve they divide into 

 tour, and produce in that shell the two i^airs of principal scars known 

 as the anterior and posterior adductors. By contraction these muscles 

 close the shell. In the ISTeotreniata they are the essential muscles, so 

 lar as scars in the fossil shells are concerned, the anterior adductors 

 closing- the valves, while the posterior pair serves to open the valves. 

 In the Atremata there is a simple pair of adductors placed near the 

 anterior extremity of the visceral area. 



Anterior region. — That portion of the shell in front of the transverse 

 axis and opposite the pedicle opening. 



Apex. — The place of initial shell growth. It may be the most jjos- 

 terior portion of the valve or may be situated near the transverse axis. 



Brachidium (Hall and Clarke). — The calcareous brachial supports of 

 the Spiriferacea and Terebratnlacea. 



Cardinal area. — A more or less Avell-develoijcd 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 fii the dorsal valve, and generally in a 

 rudimentary condition in many inarticulate species. See Deltidium. 



Cardinal extremities. — The terminations of the hinge line. 



Cardinal iwocess. — A variously moditied apophysis, situated pos- 

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

 brachiopods. 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 tiie hinge margins. 



Chilidiirm (Beecher). — A plate, in appearance similar to the del- 

 tidium, covering the exterior portion of the cardinal process in many 

 Protrenmta. Its development does not begin until early neanic or later 

 growth, and is probably secreted by the dorsal mantle lobe. 



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

 some Protrenmta, to which are attached the fleshy brachia and bra- 

 chidia. These usually form the inner walls of the dental sockets, 

 and may be sui)ported by septal plates. 



Cniralium (Hall and Clarke). — The dorsal equivalent of the A'entral 



spondylium, being formed by the convergence or union of the crural 



plates in the Pentameraeea. 



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