CONCHIFERA. 



249 



Fig. 173. Left valve . {Pecten varius) : Right valve, 



a, a, adductor; p, pedal impression; in, pallial line: I, ligamental margin; c, c, cai'- 

 tilage; e, e, anterior ears ; b. byssal sinus. 

 In the le/i valve of Jnomia there are four distinct muscular impressions 

 (fig. 175). Of these, the small posterior spot alone is produced by the ad- 

 ductor, and corresponds with the solitary impression in the right valve. 



f 



Fig. 176. Right valve. Fig. 174. Fig. 175. Left valve.* 



The adductor itself (fig. 174 «') is double. The large central impression {p) 

 is produced by the muscle of the plug (the equivalent of the byssal muscle in 

 Fim/a and BLodiola). The small impression within the umbo («) and the 

 third impression in the disk (j/) (wanting in Flacunomia) are caused by the 

 retractors of the foot. 



The term monomgary, employed by Lamarck to distinguish the bivalves 

 with one adductor, applies only to the Ostreidce, part of the Avicididce, and 

 to the genera Tridacna and Mallerla. 



The dimyary bivalves have a second adductor, near the anterior margin. 



* Fig. 176. Pi,iglit valve of ^no;wJa e/>/«p/3ZM?re, L. ^, ligamental process; 5, sinus. 

 Fig. 175, Left valve; I, ligament pit. Fig. 174. Muscular system, from a drawing 

 communicated by A. Hancock, Esq. /, the foot; pi, the plug. The muscle p\% 

 generally 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. 



