34 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Orbigny and Mytilus magellanicus Chemnitz. Some authors consider ater 

 to be the dark variety with obsolete radial ribs, mainly the more northern 

 forms, but there seems to be no foundation for this supposition. 



Aulacomya ater is an extremely variable species, like the other shallow 

 water mytilids. The young shells are yellowish, sometimes with a bluish 

 prodissoconch, and with distinct radiating ridges. The outline varies, 

 sometimes curved, sometimes with a straight ventral margin or with the 

 ventral margin convex. Older specimens become darker, with brown or 

 bluish-black periostracum. The darkest specimens with very weak radiat- 

 ing ridges seem to be found especially in the northern parts of South 

 America. 



The mantle is furnished with numerous small tentacles posteriorly. The 

 septum is usually as long as the dorsal siphonal opening and slightly 

 thickened in the middle. Some large specimens show a distinct scar for 

 the anterior adductor, while this usually is obsolete in older specimens. 

 Occurrence: This species is reported from the intertidal zone down to at 

 least 8 fms. The bottom is given as sand or sand and kelp, but apparently 

 the specimens had been fastened to stones. 



Distribution: A. ater lives from Callao, Peru, south to the Strait of 

 Magellan, and on the east coast north to southern Brazil, and in the 

 Falkland Islands. (This is not M. ater Dall (1909), Soot-Ryen (1932), 

 which probably can be referred to Mytella falcata (Orbigny).) 



Genus ISCHADIUM Jukes-Browne 1905 



Ischadium Jukes-Browne, Proc. Malacol. Soc. London, vol. 6, 1905, p. 



223. 

 Type of genus: Mytilus hamatus Say \'^22 — reciirvus Rafinesque 1820 

 (orig.). 



Remarks: Ischadium was described by Jukes-Browne (1905) as a sub- 

 genus of Brachidontes mainly because of the radiating sculpture. There 

 are, however, so many characters that separate it from Brachidontes that 

 there seems to be no reason why it should not be treated as a separate 

 genus. This genus is characterized by the broad mytiliform shape with 

 distinct radial sculpture over all the surface ; the margins are crenulated, 

 but the strong crenulations behind the ligament are absent ; the hinge 

 teeth are much like those of Mytilus s. s.; and the anterior retractor is 

 roundish behind the umbones, the posterior retractors broadly united with 

 the adductor. The mantle margins are smooth, without papillae. Appar- 

 ently the genus is monotypic. 



