180 



emission of tiie fleshy " pedicle, '' by nieuus of which tiie animal fixes 

 itself to rocks, shells, or other substances. * 



Certain genera, such as Crania, do not conform to this general method 

 of fixation, but cement their shell directly to the foreign object, while 

 others, e. g. , Pholldops, appear to have led a free existence. In many of the 

 discinoid genera, such as Orbiculoidea, etc., the pedicle passed througii 

 an opening in the lower valve; while in Lingula, the pedicle protruded 

 between the two very nearly equal valves. In all cases tlie valve giving 

 emission to the pedicle is spoken of as the pedicle valce (ventral valve 

 of many authors, dorsal valve of some). 



The opposite valve in the more specialized genera bears on its interior 

 two short processes, or crura, which arise from the hinge plate. To 

 these may be attached a calcareous " brac7iidium," which functions as a 

 support for the delicate fleshy "arms." In a large number of forms this 

 brachidium is absent, and the fleshy arms are directly supported by tlie 

 crura, but their relation to the valve in question is similar to that obtain- 

 ing in the brachidium-bearing forms. This valve is designated the 

 "bracJtiar' valve (dorsal valve of many authors, ventral valve of some). 

 In all the forms in which the valves are articulated with each other 

 (Brachiopoda articulata) such articulation is produced by teeth arising 

 from the pedicle valve, and lodged in sockets in the brachial valve. The 

 beak of the brachial valve is commonly furnished with a more or less 

 pronounced '' cardinal process," which, at its free end, presents a surface 

 for the attachment of the diductor, or opening muscles, the opposite ends 

 of which are attached near the center of the pedicle valve, where they 

 often leave pronounced sca,rs. A contraction of these muscles pulls on 

 the cardinal process, which pull, as it is exerted behind the plane of 

 articulation, will draw the beak of the brachial valve toward the interior 

 of the pedicle valve, and thus cause a separation of the valves at the 

 front or opposite end from the beak. Adductor muscles passing from 

 valve to valve, and also commonly leaving scars, close the valves again. f 

 Below the cardinal process, and often merged with it, is an elevated hinge- 

 plate, whose surface often serves for muscular attachment. 



Beneath the beak of each valve frequently occurs a flat "cardinal 

 area," bounded above by the cardinal slopes, and below by the articulating 

 margin or hinge-line. This area is commonly divided in the center by a 

 triangular fissure {delthyrium in the pedicle valve, and cJiilyriuin in 

 the brachial valve). This occurs also in genera where no ''area" is 

 present, e. g., Cryptonella, Athyris, etc. It is commonly covered either 

 by a single plate, or by two plates which join in the center. These are 

 the deltidium or pedicle plate in the pedicle valve, when single, or the 

 deltidial plates, when double, and the chilidium in the brachial valve.:): 



*This foramen frequently becomes obsolete in mature or old shells by the deposition of 

 secondary calcareous material below it. 



tin the inarticulate genera, i. e., Lingula, Orbiculoidea, etc., a more complicated nuiscu- 

 lar system exists, by means of which the valves can be partially rotated, and thus separated. 



t:In Orbiculoidea the triangular fissure is a feature of early growth only, being in the 

 later stages closed below by tlie growth of the shell. A secondary plate, or ^' listrhiiii.'' 

 also covers the apertiu'e beneath the beak. 



