488 MOLLUSCA 



T. lineata (Wood). Shell rather low, light reddish in color; surface 

 not granulated and shining; gills as in T. marmorea; length 37 mm.; 

 breadth 20 mm.: Pacific coast from Monterey to Bering Strait. 



3. Trachydermon Carpenter. Girdle covered with minute granules 

 or scales: several species. 



T. albus (L.). Shell elevated, minutely granulated, keeled; anterior 

 valve semicircular; color whitish, sometimes shading into a light orange 

 on each valve; interior white; length 15 mm.; breadth 7 mm.: circum-' 

 polar, south to Cape Cod; Pacific coast; Europe; low- water mark to 337 

 fathoms, often abundant. 



T. ruber (L.) (Fig. 750). Shell smooth, elevated, microscopically retic- 

 ulated, buff in color, marbled with reddish ; 12 mm. long and 7.5 mm. wide; 

 interior bright pink : circumpolar, south to Long Island Sound and Sitka ; 

 Europe; from low water mark to 80 fathoms; rare south of Cape Cod. 



4. IscHNOCHiTON Gray. Girdle covered with imbricating scales: 

 numerous species. 



I. magdalenensis (Hinds). Shell elongate and rather narrow, faintly 

 mottled with olive on a light ground; central areas pitted; lateral areas 

 and end valves with radiating lines ; lengih 75 mm. ; breadth 

 30 mm.: Monterey, California, to Magdalena Bay. 



5. NuTTALLiNA Carpenter. Valves exposed, granulated, 

 the lateral areas having 2, the head valve numerous low 

 radiating ribs; girdle varying from spiny to scaly; umbo 

 behind the middle: several species. 



N. calif ornica (Nuttall) (Fig. 751). Shell more than 



twice as long as wide, dark brown in color, with whitish 



streaks; median valves strongly beaked with a smooth 



rounded dorsal ridge ; girdle covered with minute spines ; length 37 mm. ; 



breadth 15 mm.: coast of Oregon, Washington, and California. 



Family 2. MOPALIIDAE. 



Valves of large size on a broad body; each intermediate valve with 

 a single slit; posterior valve with a median sinus; insertion plates on all 

 the valves; girdle hairy, never scaly; gills extend the length of the foot: 

 3 genera; Pacific Ocean. 



MoPALiA Gray. Valves exposed; insertion plate of anterior valve 

 slit into nearly smooth teeth; insei-tion plate of posterior valve with an 

 oblique slit on each side: 5 species, all on Pacific coast. 



M. muscosa (Gould). Shell oval, brownish in color, but sometimes 

 bright green or red, anterior valve having about 10 radiating ribs; length 

 50 mm. ; breadth 3.8 mm. : coast of Washington, Oregon, and California, 

 between tide lines. 



