CONCHirEEA. 



403 



impression. The Unionidce haye two additional retractors of the 

 foot, attached laterally behind the anterior adductors ; in Leda, 

 Solenella, and a few others, this lateral attachment forms a 

 line extending from the anterior adductor backwards into the 

 umbonal region of the shell. (See PI. XYII., Tigs. 21, 22.) 



In those shellfish like Pinna and the mussel, which are per- 

 manently moored by a strong hyssus, the foot (/) serves only to 

 mould and fix the threads of which it is formed. The fibres 

 of the foot-muscles pass chiefly to the bj^ssus (5), and besides 

 these two additional muscles (pp) are developed. In Pi7ina, 



Fig. 214. Muscles of Modiola.* 



Modiola, and. Dreissena the byssal muscles are equal to the great 

 adductors in size. 



In a few rare instances the muscles are fixed to promi- 

 nent apophyses. The falciform processes of Pholas and Teredo 

 (PI. XXIIL, Figs. 19, 26) are developed for the attachment of 

 the foot-muscle ; the posterior muscular ridge of Diceras and 

 Cardilia resembles a lateral tooth, and in the extinct genus 

 Radiolites both adductors were attached to large tooth-like pro- 

 cesses of the opercular valve ; but, as a rule, the muscles deposit 



* Fig. 214. Muscular system of 31odioIa modiolus,!:, from a drawing communi- 

 cated by A. Hancock, Esq. act, anterior, a'a', posterior adductors ; uu and p'p', pedai 

 muscles ; pp, byssal muscles ; /, foot ; b, byssus ; vr, pallial line. 



