CONCHIFERA. 403 
impression. The Unionide have 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 Pl. XVII., Figs. 21, 22.) 
In those shellfish like Pinna and the mussel, which are per- 
manently moored by a strong byssus, 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 (0), and besides 
these two additional muscles (p p) are developed. In Pinna, 
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 
(Pl. 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-lhke pro- 
cesses of the opercular yalye; but, as a rule, the muscles deposit 
* Fig. 214. Muscular system of Modiola modiotus, L., from a drawing communi- 
cated by A. Hancock, Esq. aa, anterior, a'a', posterior adductors; wu and p'p', pedal 
muscles ; pp, byssal muscles ; 7, foot; 0, byssus; m, pailial line, 
