32 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Remarks: This is a remarkable species, as the retractor muscles are ex- 

 tremely strong and the tentacles on the mantle margin very large. The 

 posterior retractors are only narrowly connected with the adductor dor- 

 sally, and the muscle-bundles lie almost horizontally toward the byssal 

 groove, which is in the anterior third of the shell. The anterior retractor 

 is placed below the ligament in the anterior third of the valves. The 

 tentacles on the posterior part of the mantle are formed as broad stems 

 with three to five branches, each furnished with numerous papillae. Be- 

 tween the main stems is one small tentacle, like one branch of the large 

 ones. The dorsal siphon is long and slightly protruding. 



The byssus is very strong. From a solid, round central stem, strong 

 round branches extend to all sides. The foot is small, flattened below and 

 furnished with a continuous slit. Behind the foot the visceral mass is seen 

 penetrating halfway down into the mantle cavity. 



The shell ordinarily is very worn, with the dark periostracum pre- 

 served on the younger parts only. The shell substance is worn off in several 

 planes, forming different irregular keels on the anterior part. The ventral 

 margin is bent slightly inward anteriorly. 



Young Choromytilus palliopunctatus are sometimes very like small 

 Semimytilus algosus (Gould), as they have a small anterior adductor; but 

 the large anterior retractor easily separates them. 



Occurrence: This species seems to be intertidal, living fastened to rocks 

 and apparently on exposed coasts. The strong byssus and the worn shells 

 seem to indicate that they are able to withstand heavy surf. 

 Distribution: Bahia de la Magdalena, Baja California, to Puerto Pinas, 

 Panama. 



Genus AULAGOMYA Morch 1853 



Aulacomya Morch, Catalogus Conchyliorum quae reliquit D. Alphonso 

 d'Aguirra & Gadea, comes de Yoldi. Fasc. secundus. Acephala, 1853, 

 p. 53. 

 Type of Genus: Mytilus magellanicus Chemnitz 1785 p. p. = Mytilus 



ater Molina 1782 (subsequent designation by Ihering 1900). 

 Remarks: Under Aulacomya nob. Morch gave two species, magellanicus 

 Chemnitz and crenatus Lamarck, Enc. PI. 217 f. 3, both listed as from 

 "Am. mer." and considered to be conspecific. Though M. magellanicus 

 Chemnitz contained originally also Mytilus exustus Linne, the designa- 

 tion of magellanicus as the type of Aulacomya seemed to be valid. 



