402 MANUAL OF THE MOLLUSCA. 
muscles themselves may be composed of two elements,* as 
in Oytherea chione (Fig. 14, p. 20) and the common oyster. 
The impression of the posterior adductor in Spondylus is double 
(Pl. XVI., Fig. 15). In Pecten varius (Fig. 210, a a), large 
independent impressions are formed by the two portions of 
the adductor, and in the left valve there is a third impression 
(p) produced by the foot, which in the byssiferous pectens is a 
simple conical muscle with a broad base. 
In the left yalve of Anomia there are four distinct muscular 
impressions (Fig. 213). Of these, the small posterior spot alone 
is produced by the adductor, and corresponds with the solitary 
Fig. 211. Right valve. Fig. 212, Fig. 213, Left valve.t 
impression in the right valve. The adductor itself (Fig. 212, ) 
is double. The large central impression (p) is produced by the 
muscle of the plug (the equivalent of the byssal muscle in Pinna 
and Modiola). The small impression within the umbo (u) and 
the third impression in the disc (p') (wanting in Placunomia) are 
caused by the retractors of the foot. 
The term monomyary, employed by Lamarck to distinguish 
the bivalves with one adductor, applies only to the Ostreide, 
part of the Aviculide, and to the genera T'ridacna and Miilleria. 
The dimyary bivalves have a second adductor, near the anterior 
margin, which is small in Mytilus (Fig. 30), but large in Pinna. 
The retractor muscles of the foot (already alluded to at p. 20) 
have their fixed points near those of the adductors; the anterior 
pair are attached within the umbones (Fig. 214, wu), or nearer 
the adductor, as in Astarte and Unio (Fig. 209). The posterior 
pair (p’y’) are often close to the adductor, and leaye no separate 
* Compare the shell of modiola, Pl. XVII., Fig. 5, with the woodcut, Fig. 214. 
7 Fig. 211. Right valve of Anomia ephippium, L. 1, ligamental process; s, sinus 
Fig. 213. Left valve; 7, ligament pit. Fig. 212. Muscular system, from a drawing 
communicated by A. Hancock, Esq. f, the foot; pl, the plug. The muscle p is 
generaily described as a portion of the adductor ; but it is certain, from a comparison 
of this shell with Carolia and Placuna, that a' represents the entire adductor, and p 
' the byssal muscle. 
