210 



MANUAL OF THE MOLLUSCA. 



hinge ; in Crania and Discina the lower valve is flat, the upper like a limpet ; 

 the valves of Lingula are nearly equal, and have been compared to a duck's 

 bill. (Petiver). 



Ventral valve. 



Dorsal valve. 



Fig. 109. Muscular system of Terebratula * 



a. a. adductor-muscles; r. cardinal-muscles; x. accessory cardinals; p. ventral 

 pedicle-muscles ; p.' dorsal pedicle-muscles ; z.capsular-muscles; o. mouth; v. vent; 

 /. loop ; t. dental socket. 



The valves are both opened and closed by muscles ; those which open 

 the shell (cardinales) originate on each side the centre of the ventral valve, 

 and converge towards the hinge-margin of the free valve, behind the dental 

 sockets, where there is usually a prominent cardinal process.^ The teeth 

 form the fulcrum on which the dorsal valve turns. The adductor muscles 

 are four in number, and quite distinct in Crania and Discina ; in Linyula 

 the posterior pair are combined, and in Terebratula the four muscles are 

 separate at their dorsal terminations, but united at their insertion in the 

 centre of the larger valve. The pedicle is fixed by a pair of muscles (each 

 doubly -attached) to the dorsal hinge-plate, and by another pair to the ventra 

 valve, outside the cardinal muscles.J In the hinge-less genera the contrac- 

 tion of the cardinal muscles must tend to slide the free valve forwards, and 

 in Crania and Discina these muscles are attached to a prominent ventral 



* Waldheimia Australia, Quoy. f . From a drawing by Albany Hancock, Esq. 



t The term " retractors' used at p. 8 is relinquished for the more appropriate 

 term " cardinal muscles,'" given by Prof. King. They are particularly interesting 

 from their function, as antagonists of the adductor muscles, like the ligament of 

 ordinary bivalves. 



J The muscular system of Terebratula presents a considerable amount of resem- 

 blance to that of Modiola (fig. 177); the anterior and posterior pedal muscles may be 

 compared to the dorsal and ventral pedicle muscles. 



