COIS'CHIFERA . 



403 



impression. Tlie Uniomdce have two additional retractors of the 

 foot, attaclied laterally behind the anterior adductors ; in Lcda, 

 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., Eigs. 21, 22.) 



In those shellfish like Pinna and the mussel, which are ]3er- 

 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 byssus (5), and besides 

 these two additional muscles [jp Jp) are developed. In Pinna y 



Fig. 214. Muscles of Madiola. * 



Ilodiola, and Breissena the byssal muscles are equal to the great 

 adductors in size. 



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

 nent apo2:)hyses. The falciform processes of Pholas and Teredo 

 (PI. XXIII. , 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 Modiola modiohis, L.. from a drawing communi- 

 cated bj' A. Hanc' ck, Esq. aa, anterior, a'a\ posterior adductors ; uu and p'p', pedai 

 muscles : vp, byssal muscles ; /, foot ; b, byssus ; m, pallial line. 



