402 



MANUAL OF THE MOLLUSCA. 



muscles themselves may be composed of two elements,* as 

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

 The impression of the posterior adductor in Spondylus is double 

 (PL XVI., 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 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 valve 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. 



Fiff. 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 hyssal muscle in Pinna 

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

 the third impression in the disc {p') (wanting in Placunoviia) 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 Ostreidce, 

 part of the AvicuUdts, and to the genera Tridacna and Miilleria. 



The dimyary bivalves havO a second adductor, near the anterior 

 margin, which is small in Mytilas (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, u u), or nearer 

 the adductor, as in Astarte and Um'o (Fig. ?09). The posterior 

 pair {p'p) are often close to the adductor, apd leave no separate 



* Compare the sliell of modiola, PL XVII., Fig. 5, with the woodcut, Fig. 214. 



T Fig. 211. Eight valve of Anomia ephippium,L. Z, ligapiental process; s, sinus. 

 Fig iSl3. Left valve; /, ligament pit. Fig. 212. Muscular system, from a drawing 

 comjnunicated by A. Hancock, Esq. f, the foot; pi, the plug. The muscle;; ia 

 generally described as a portion of the adductor ; but it is certain, frojii a comparison 

 of li isi fill ell with Carojia and Placujfa, that a' represents the entire adductor, andp 

 the b^'Hsal muscle. 



