NO. 1 



SOOT-RYEN : THE FAMILY MYTILIDAE 



73 



Fig. 60. Amygdalum pallidulum (Dall). Santa Cruz Island, Cali- 

 fornia. Adductor and retractor muscles. 



Fig. 61. Lioberus salvadoricus (Hertlein and Strong). Bahia Cocos, 

 Costa Rica. Note the place of the anterior retractor dis- 

 tinctly before the umbo. 



Fig. 62. Lioberus salvadoricus (Hertlein and Strong). Bahia Cocos, 

 Costa Rica. The siphons, showing that the branchial siph- 

 on, not closed ventrally, is of the same length as the anal 

 siphon. 



Fig. 63. Musculus discors (Linne). Ireland. Arrangement of muscle 

 scars. 



Fig. 64. Gregariella coarctata (Carpenter). Laguna de Scammon, 

 Baja California. Posterior part seen from behind. 



Fig 65. Gregariella chenui (Recluz). Seal Beach, California. A 

 branched periostracum hair. 



Species with a quite different anatomy have, because of the form and 

 sculpture, been placed in the genus Musculus. These species, e. g., M. 

 marmoratus (Forbes), M. impactus (Hermann), M. cumingianus 

 (Reeve), M. lebourae White, are more inflated than the typical Muscu- 

 lus, and the arrangement of the retractor muscles is quite different. (For 

 the anatomy of M. marmoratus, see List, 1902, PI. 20, figs. 4-5 ; and for 

 that of M. lebourae, see White, 1949, p. 47.) The anterior retractor is 

 like Musculus, but the posterior retractors have a strong, elongated 

 branch fastened along the dorsal margin and a single slender branch 

 fastened above the posterior adductor. 



