402 



MANUAL OF THE MOLLTJSCA. 



muscles themselves may be composed of two elements,* as 

 in Cytherea chione (Fig. 14, p. 20) and the common oyster. 

 The impression of the posterior adductor in Spondylus is double 

 (PI. Xyi., Fig. 15). In Feden varius (Fig. 210, a a), large 

 independent impressions are formed by the two portions of 

 the adductor, and in the left yalye there is a third impression 

 {jj) produced by the foot, which in the byssiferous pectens is a 

 simple conical muscle with a broad base. 



In the left yalye of Anomia there are four distinct muscular 

 imj)ressions (Fig. 213). Of these, the small posterior spot alone 

 is produced by the adductor, and corresponds with the solitary 



Fiff. 211. Eight valve. 



Fig. 212. 



Fig. 213. Left valve.t 



impression in the right yalye. The adductor itself (Fig. 212, a) 

 is double. The large central impression (p) is produced by the 

 muscle of the plug (the equiyalent of the lyssal muscle in Pinna 

 and Modiola). The small impression within the umbo [u) and 

 the third impression in the disc [p') (wanting in Placimomia) are 

 caused by the retractors of the foot. 



The term monomyary, employed by Lamarck to distinguish 

 the bivalyes with one adductor, applies only to the Osfreidce, 

 part of the Aviculidce, and to the genera Tridacna and Millleria. 



The dimyary biyalyes haye a second adductor, near the anterior 

 margin, which is small in Mytilus (Fig. 30), but large in Pinna. 

 Th.Q . retractor muscles of the foot (already alluded to at p. 20) 

 haye their fixed points near those of the adductors ; the anterior 

 pair are attached within the umbones (Fig. 214, u u), or nearer 

 the adductor, as in Astarte and Unio (Fig. 209). The posterior 

 pair [x^' p) are often close to the adductor, and leaye no separate 



* Compare the shell of modiola, PI. XVII., Fig. 5, with the woodcut, Fig. 214. 



T Fig. 211. Eight valve of Anomia ephippiicm, L. /, ligamental process ; s, sinus. 

 Fig. i2i;3. Left valve; I, ligament pit. Fig. 212. Muscular system, from a drawing 

 communicated by A. Hancock, Esq. f, the foot ; pi, the plug. The muscle p is 

 genera Jy described as a portion of the adductor ; but it is certain, from a comparison 

 of tl is fihcll with Carolia and Placuna, that a' represents the entire adductor, andp 

 the byssal muscle. 



