38 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



hand, but it is to be hoped that students will try to settle the problem in 

 the future. There has been much discussion about this species, both in 

 regard to the distinction of it and the proper name for it. Dunker's de- 

 scription fits one form only and should not be used for the many other 

 heavily sculptured forms. The real Hormornya adamsiana should be of a 

 dark purplish color and furnished with bifurcating ribs elegantly granu- 

 lated. Pilsbry and Raymond (1898) introduced the name Mytilus 

 stearnsi for the California species called Mytilus bifurcatus and in 

 appearance very like Septifer bifurcatus (Conrad), with the type locality 

 for M. stearnsi given as San Diego. In spite of these disagreements, I 

 think it safest at the present time to use adamsiana Dunker for the vari- 

 able complex of Hormomyas living along the Pacific coast of America 

 from Santa Barbara, California, to Ecuador. There might be a possibility 

 that Dunker's name should be used for another species, perhaps for 

 Mytilus puntarenensis Pilsbry and Lowe, 1932, which has to be placed 

 in the genus Brachidontes. 



Fig. 21. Hormomya exusta (Linne). Long Key, Florida. Hinge of 



left and right valves. 

 Fig. 22. Hormomya adamsiana (Dunker). Isthmus Cove, Santa 



Catalina Island. Showing insertion of retractors in the 



posterior adductor. 

 Fig. 23. Hormomya sp. Sculpture on the ventral side, showing 



unilateral furcation, a. Enlarged part. 

 Fig. 24. Brachidontes sp. Sculpture on the ventral side showing 



bifurcation, a. Enlarged part. 

 Fig. 25. Hormomya adamsiana (Dunker). Isthmus Cove, Santa 



Catalina Island, California. Hinge. 

 Fig. 26. Hormomya granulata (Hanley). Bahia de la Inde- 



pendencia, Peru. Hinge. 

 Fig. 27. Brachidontes modiolus (Linne). Lower Matecumbe, 



Florida. Hinge of left and right valves. 

 Fig. 28. Brachidontes multiformis (Carpenter). Mazatlan, Sinaloa, 



Mexico. Hinge. 

 Fig. 29. Brachidontes puntarenensis (Pilsbry and Lowe), Punta 



Arenas, Costa Rica. Paratype. Hinge. 

 Fig. 30. Brachidontes purpuratus (Lamarck). Bahia de la Inde- 



pendencia, Peru, Hinge. 

 Fig. 31. Hormomya adamsiana (Dunker). Puerto Parker, Costa 



Rica. Mantle with dorsal opening, septum, and the small 



mantle papillae seen from behind. 

 Fig. 32. Brachidontes modiolus (Linne). Lower Matecumbe, 



Florida. Posterior part with tentacles on the mantle. 

 Fig. 33. Septifer bifurcatus (Conrad). San Pedro, California. In- 

 side of right valve with the anterior septum for the ad- 

 ductor muscle, and the posterior adductor embracing the 



retractors. 

 Fig. 34. Septifer zeteki Hertlein and Strong. Off Colombia. Hinge 



and septum of right valve. 



