BRACHIOPODA. 437 



instead of being dorsal and ventral. Further, the two valves 

 are usually of the same size (" equivalve "), and a line drawn 

 from the beak to the base would almost always divide the 

 shell into unequal halves; so that the shell is "inequi- 

 lateral." 



Ordinarily the ventral valve of the shell of the Brachio- 

 pods is the larger of the two, and it is generally furnished 

 with a prominent curved " beak." Very commonly the beak 

 as perforated by a larger or smaller aperture, which is termed 

 the "foramen" (fig. 275, B), and which serves for the trans- 

 mission of a muscular peduncle or stem by which the shell 

 is attached to some foreign object. In some cases, however 

 (as in Lingula, fig. 274, B), the peduncle simply passes be- 

 tween the apices of the valves, and there is no foramen ; 

 whilst in others (as in Crania, fig. 274, 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. 



In most living Brachiopods the valves are articulated to 

 one another by two teeth which are developed upon the 

 ventral valve, and fit into corresponding sockets in the dor- 

 sal valve. Behind the dental sockets of the dorsal valve 

 there is usually a prominent process (" cardinal process "), to 

 which are attached two " cardinal muscles." These are in- 

 serted on each side of the centre of the ventral valve, and 

 serve to open the shell. The valves of the shell are closed 

 by proper " adductor muscles " (usually four in number), 

 which also pass between the valves ; and in those in which 

 hinge-teeth are wanting, it is by these muscles that the 

 valves are kept together. These muscles leave " scars " or 

 impressions at their points of insertion and origin : and the 

 number and form of these scars afford important diagnostic 

 characters to the palaeontologist. 



Very commonly, the beaks of the dorsal and ventral 

 valves are separated from one another by a narrower or 

 wider space, which is termed the "hinge-area" (fig. 276). 

 In some genera, as in Spirifera, the area is very conspicu- 



