16 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



SYSTEMATICAL PART 



KEY TO GENERA OF MYTILIDS 

 BASED ON WEST AMERICAN SPECIES 



1. Resilial ridge pitted 2 



Resilial ridge compact 4 



2. No anterior adductor, posterior retractor scars widely sepa- 



rated Perna 



Anterior adductor present 3 



3. Anterior retractor scar elongate, posterior retractors continu- 



ous, several small teeth present Mytilus 



Anterior retractor scar round, posterior retractors separated, 

 anterior margin with or without teeth ; more or less green 

 colored Mytella 



4. Lunule with radial sculpture and anterior margin with teeth 



or crenulations 5 



Lunule smooth, anterior margin without teeth or crenulations, 



shell smooth or with irregular sculpture 18 



5. Shell with concentric or irregular sculpture, or smooth .... 6 

 Shell with radial sculpture though sometimes obsolete .... 9 



6. Anterior retractor before umbo, scars of siphonal muscles vis- 



ible ; shell elongate lithophagiform or modioliform, smooth 



or with irregular sculpture Adula 



Anterior retractor elongate, behind umbo, shell mytiliform ... 7 



7. 1 to 3 distinct teeth or folds 8 



No real teeth, lunule bordered by a faint line . . . Semimytilus 



8. Anterior adductor strong, shell margin minutely 



crenulated Crenomytilus 



No anterior adductor, shell margin smooth .... Choromytilus 



9. Strong tooth-like crenulations behind the ligament 10 



Dorsal margin behind ligament smooth or crenulated like the 



rest of the margins 12 



10. Anterior adductor placed on a septum in each valve . . . Septifer 

 No anterior septum 11 



11. Umbones subterminal, anterior hinge margin bent in an angle, 



radial sculpture on ventral part simple or regularly bifur- 

 cating, posterior retractors fastened to dorsal part of poste- 

 rior adductor Brachidontes 



