BRACHIOPODA. 



645 



Ordinarily, the ventral valve of the shell of the Brachiopods is the 

 larger of the two, and it is generally furnished with a prominent 

 curved "beak." Very commonly the beak is perforated by a larger 

 or smaller aperture, which is termed the "foramen" (fig. 490, b), 

 and which serves for the transmission of a muscular peduncle or 

 stem by which the shell is attached to some foreign object. In 

 some cases, however (as in Lingitla, fig. 486, b), the peduncle 

 simply passes between the apices of the valves, and there is no 

 foramen; whilst in others (as in Crania, fig. 486, d), the shell is 

 merely attached by the substance of the ventral valve. The dorsal 

 valve, which is also usually the smaller, is always free, and is never 

 perforated by a foramen. Further, as already remarked, there is 

 reason to believe that some fossil forms were free and unattached 

 in their adult condition. 



The B?'achiopoda are divided into two primary sections according 

 as the valves are held together by muscular action alone, and there 

 is no " hinge " developed (" Inarticulata "), or as the valves are pro- 



Fig. 491. — Muscular system of WaldheimiaJIavescens. M, Ventral^ valve ; N, Dorsal valve ; 

 /, Loop ; v, Mouth ; z, Extremity of intestine ; a a, Adductor ; c, Divaricators ; c, Accessory 

 divaricators ; b, Ventral adjustors ; b' ', Peduncular muscle ; b", Dorsal adjustors ; p, Peduncle. 

 (After Hancock.) 



vided with a proper "hinge" (" Articulata"). In the Articulated 

 Brachiopods the valves are united at their beaks by means of two 

 teeth which are developed on the ventral valve, and fit into cor- 

 responding sockets in the dorsal valve. Behind the dental sockets 

 of the dorsal valve there is usually a prominent process (" cardinal 

 process "), to which are attached the so-called " cardinal " or " divari- 

 cator" muscles (fig. 491, c). These are inserted on each side of 

 the centre of the ventral valve, two pairs being usually present ; and 

 they serve to open the shell, no structure corresponding with the 



