116 AXEL A. OLSSON 



Shell long, swollen, rather straight, pointed, almost always rubbed at 

 various angles, but when perfect exhibiting fine radiating stria and some^ 

 times much stronger concentric lines of growth. Shell reddish purple in 

 color, often white gray on the surface. Interior of a lustrous dark purple, 

 white near the byssal gap and often finely punctate. The beak is terminal, 

 with the anterior margin deeply impressed below to produce a folded hinge 

 structure above it. The posterior adductor is relatively large, pipe-shaped, 

 the retractor scar placed under the resilifer about half way towards the beak. 

 No anterior adductor scar. 



Range — Lower California to Panama. Mexico: Mazatlan; Acapulco. 

 Nicaragua: Corintho (Hertlein and Strong). 



Genus Aulacomya Morch, 1853 



Type species by subsequent designation, Von Ihering, 1900, Mytilus 

 magellanicus Chemnitz, 1785 (= Mytilus ater Molina, 1782). Straits of 

 Magellan and northward to middle Peru. 



Shell large, mytiliform and often coarse, the surface marked with 

 strong, radiating striae over most of its disk or confined to the 

 umbonal slope. Beaks small and terminal. The hinge which is placed below 

 and behind the beak is formed by a plate continued in from an obscure 

 lunule and consists of a strong rib or fold in the right valve and a flattened 

 furrow in the left valve. The resilifer is long, narrow, chalky, and compact 

 or with an unpitted foundation. Anterior adductor scar eliminated in the 

 adult. Cold waters of the Southern Hemisphere. 



Aulacomya ater (Molina) Plate 14, figure 9 



Mytilus magellanicus Chemnitz, 1785, in Martini and Ciiemnitz, Neues Syst. Conchyl.- 

 Cabinet, vol. 8, p. 162, t. 83, fig. 742.— Lamarcic, 1819, Anira. s. Vert, vol. 6, 

 pt. 1, p. 119.— Lamarcic, Encyclop. Meth., pi. 217, fig. 2.— Dall, 1909, Proc. 

 U. S. Nat. Museum, vol. 37, No. 1704, pp. 151, 258, pi. 25, fig. 4. 



Mytilus decussatus Lamarck, 1819, Anim. s. Vert., vol. 6, pt. 1, p. 120. — Hidalgo, 1879, 

 Moluscos del Viaj« al Pacifico, p. 52, pi. 3, fig. 4. 



Mytilus americanus d'Orbigny, 1846, Voy. Amer. Merid., vol. 5, Mollusques, p. 648. 



Mytilus ater Molina, 1782, Saggio sulla Storia Naturale del Chili, p. 202. 



Aulacomya ater (Molina), Soot-Ryen, 1955, Allan Hancock Pacific Expeditions, vol. 

 20, No. 1, pp. 33, 34, pi. 1, fig. 6; text-figs. 17-18. 



Along the Peruvian coast, this species has been reported as far north 

 as Callao. It appears to be plentiful at Paracas and southward. Fresh 

 shells have a dark-brown, polished periostracum which in old shells peels 

 off, revealing a purplish-white surface streaked with radial lines of purple 

 over the whole disk or confined to the umbonal slope; interior pearly white. 



Range — Southern half of the South American continent on both coasts, 

 and on the Pacific side as far north as Callao. 



Genus PERUMTTILUS new genus 



Type species Modiola purpurata Lamarck. 



Shell of medium size, with obliquely elongated, convex valves, the 

 beaks nearly terminal. Sculpture produced by relatively large, crude radial 



