NO, 1 SOOT-RYEN : THE FAMILY MYTILIDAE 69 



This genus comprises species from deeper water apparently not closely 

 related to Modiolus, perhaps closer to Mytella. As the species occur in all 

 oceans and as the specific diiiferences seem to be small, there has been a 

 tendency to put all specimens in two species of world wide distribution. 

 This seems not to be justified. There is a slight but distinct difference 

 between the Atlantic Amygdalum politum (Verrill and Smith) in Verrill 

 1880 and the Pacific Amygdalum pallidulum (Dall) 1916 in the color 

 and spots on the dorsal part. The Atlantic Amygdalum dendriticum 

 Megerle von Miihifeld \^\\=arborescens (Chemnitz) 1795, is broader 

 posteriorly like the broader Pacific species, and is furnished with mark- 

 ings of a pattern dififerent from the narrower Pacific species Amyg- 

 dalum beddomei Pettard and from species close to A. beddomei found on 

 the west coast of America between Isla Gorgona, Colombia, and Payta, 

 Peru. These specimens are here supposed to constitute an unrecognized 

 species very close to but not identical with the Tasmanian A. beddomei. 

 Key to the west American species : 



Shell much higher posteriorly than anteriorly ; 



white lines parallel to dorsal margin and 



scattered white spots on a hyaline shell on the 



posterodorsal part pallidulum (Dall) 



Shell slightly higher posteriorly, grayish- 

 brown triangular spots and irregular blotches 



on the posterodorsal part americanum n. sp. 



Amygdalum pallidulum (Dall) 1916 



Plate 8, fig. 36; text-fig. 60 



Modiolus pallidulus Dall, Proc. U. S. Natl. Mus., vol. 52, 1916, p. 404. 

 Holotype: U. S. National Museum. Cat. no. 212746. 

 Type loc: Of? San Luis Obispo Bay, California; 77 fms. 

 Remarks: The shell is elongate and rather narrow, with the nearly 

 straight dorsal and ventral margins forming an angle of about 30°. The 

 umbones are placed slightly behind the rounded anterior margin, which 

 shows faint radiating lines on the lunular part. The posterior margins 

 are evenly rounded. The anterior and central areas are opaque, grayish- 

 white with yellow stains; the dorsal and posterior areas are translucent 

 and furnished with white markings, which form longitudinal stripes dor- 

 sally and rhomboidal meshes, which sometimes become partly obsolete, 

 on the central and posterior parts. The muscle scars are very indistinct 

 on the thin shell, but the muscles are easily seen in specimens with the 

 soft parts. 



