20O 



MOLLUSCA. 



forated by a larger or smaller aperture, which is termed the 

 " foramen " (fig. 139, 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 Lingula\ 

 the peduncle simply passes between the apices of the valves, 

 and there is no foramen ; whilst in others (as in Crania, fig. 

 161), the shell is merely attached by the substance of the ven- 

 tral valve. The dorsal valve, which is also usually the smallest, 

 is always free, and is never perforated by a foramen. Further, 

 as already remarked, there is reason to believe that many 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 dorsal 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 inserted 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 impor- 

 tant 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. 140). In some genera, 



. Terebratella Astieriaiia, Cretaceous, e Hinge-area; m Ueltidium. 



as in Spirifera, the area is very conspicuous ; in other cases it 

 is very narrow, or even does not exist. In front of the foramen 

 of the ventral valve, and very often forming part of its circum- 

 ference, there is commonly a triangular plate, which may be 



