NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 83 



The shells usually have mud and sand particles adhering to the surface, 

 and on both sides of the umbones there is a ferruginous coating. The 

 anatomy of this species is most interesting and unlike that of other mytil- 

 ids. The anterior adductor is very narrow and elongate and placed along 

 the extreme border of the anteroventral angle. The posterior adductor is 

 seen as a round dark spot in the posterodorsal part, a little inside the shell 

 margin. The foot is strong, cylindrical, with a swollen apex furrowed on 

 the posteroventral side and showing a median constriction. There seems 

 to be no open furrow on the posterior part of the foot proper and no traces 

 of a byssus or a byssal gland. Two short anterior retractors are fastened 

 to the valves in the umbonal cavity, and thin muscle threads extend back- 

 ward to the posterior adductor and are fastened to the shell or the adduc- 

 tor on the sides. There are no muscles for the byssus, and the foot retrac- 

 tors are astonishingly weak. The form of the foot indicates that the 

 animal moves in the mud by anchoring the foot and inflating the apex 

 like a balloon ; then the animal is pulled forward by the weak muscles. 

 The mantle margins are united in the anterior half of the ventral side. 

 The ventral opening is short and is closed posteriorly by the septum, 

 which is striated longitudinally. Just before the septum opens to a rela- 

 tively large anal opening, the margins are furnished with a row of small 

 papillae. The gonads are placed on the dorsal side. 



Occurrence: This species is found in depths varying from 16 to 290 fms. 

 The bottom usually consists of mud of various types or of sand; also 

 gravel is reported from a few localities. 



Distribution: The distribution has been given as from the Aleutian 

 Islands to San Diego, California. The southern limit is now moved to 

 Isla Clarion, Mexico. It is also found in the Gulf of California. If we 

 include megas Dall, the southern limit is the Bahia de Panama. 



Genus BOTULA Morch 1853 



Botula Morch, Catalogus Conchyliorum quae reliquit D. Alphonso 

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

 p. 55. 

 Type of genus: Mytilus fuscus Gmelin 1791 (Subsequent designation by 



Dall, Bartsch, and Rehder 1938). 

 Remarks: This genus is rather different from all other groups studied, 

 as the anterior retractor is placed on the margin of the shell just below the 

 umbones. The posterior adductor is small and the posterior retractor 



