66 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL, 20 



Remarks: The periostracum is hairy sometimes over most of the surface 

 but always on the posterior part ; the hairs are fine, often with a narrow 

 lateral velum, and coarsest on the posterior side. 



The anterior lunular part, so characteristic in most of the specimens, 

 is not so well defined in small specimens, which often look very like small 

 M. modiolus. The lunule is dull and usually distinctly set off from the 

 rest of the shell. The inside is margaritaceous, white or reddish-white; 

 the shell is thin compared to M. capax and M. fornicatus. The ligament 

 is short, and the umbones nearly touch in the middle. 



The anterior adductor is placed along the anterior part of the ventral 

 margin, the posterior adductor is strong, with the retractors narrowly 

 separated in two series, the anterior retractor strong. The posterior mantle 

 margin is thick and protruding, the septum short with an inwardly bent 

 median flap, and the dorsal opening large. The foot is strong, somewhat 

 flattened; and the labial palpae are long. 



Occurrence: Apparently a deeper water species living in "nests" of sand 

 or hidden among holdfasts, etc. 



Distribution: Hitherto recorded from Bechers Bay, California, south to 

 San Clemente Island, California. 



Modiolus modiolus (Linne) 1758 

 Text-figs. 47, 48, 52 



Mytilus modiolus Linne, Systema Naturae, ed. 10, 1758, p. 706. 

 Halo type: ? 



Type loc: The Mediterranean. 



Remarks: Typical specimens of this species, which is very common in the 

 north Atlantic, are found in the northern parts of the Pacific. Smaller 

 specimens have strong but smooth hairs on the periostracum, but these 

 hairs are usually lost in older specimens. The anterior part is rounded 

 and protrudes slightly beyond the umbones. The rather large lunule is 

 dull and usually distinct in smaller specimens. The outline is rather 

 variable and sometimes is much like some forms of M. americanus 

 (Leach) or M. capax (Conrad). The posterior part of the mantle is 

 much like that of 71/. capax, but the margins are more protruding and the 

 septum longer, with a more pronounced median flap. 



There has been much discussion about the distribution of AI. modiolus 

 on the west coast. Usually the southern limit is stated to be Laguna de 

 San Ignacio, Baja California, but Burch, quoting Myra Keen, favors a 

 southern limit at Monterey (see discussion in the Minutes of the Concho- 

 logical Club of Southern California, no 36, June, 1944, p. 12). The rec- 



