NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 91 



Occurrence: Boring in soft shale, but now and then found fastened by 

 the byssus to stones, indicating that this species does not always live 

 as a borer. Generally living in the intertidal zone. 

 Distribution: Vancouver Island to San Diego, California; Japan. 



Adula diegensis (Dall) 1911 

 Plate 9, fig. 51 ; text-figs. 75-76 



Modiolus diegensis Dall, Nautilus, vol. 24, 1911, pp. 110-11. 



Holotype: U. S. National Museum. 



Type loc: San Diego, California. 



Remarks: Dall described this species as a Modiolus but said it resembled 



Adula, though not a borer, and it might possibly be referable to the genus 



Myrina H. and A. Adams. It seems, however, to have its correct place 



in the genus Adula. 



The shell is smooth except on the lunule, where six radiating folds 

 are seen, being especially distinct on the inside and forming teeth on the 

 anterior margin. The periostracum is often covered by a filthy layer 

 posterodorsally. The anatomy agrees with that of other species of 

 Adula. The posterior extensible part of the mantle is furnished with 

 papillae like those found in Adula calif orniensis. 



Occurrence: Found on mud flats or piling, apparently always free-living. 

 Intertidal. 



Distribution: Charleston, Oregon, south to Cabo San Lucas, Baja Cali- 

 fornia. La Libertad, Sonora, Mexico (Lowe collection, San Diego 

 Museum). 



Genus LITHOPHAGA Roding 1798 



Lithophaga Roding, Museum Boltenianum, Part II, 1798, p. 156. 

 Syn.: Lithophagus Megerle von Miihlfeld 1811. 



Lithodomus Cuvier 1817. 



Dactylus Lang 1722, Klein 1753, Morch 1861. 

 Type of genus: Lithophaga mytuloides Roding 1798 = Mytilus lithopha- 

 gus Linne 1780. 



Remarks: Species belonging to this genus are elongate with more or less 

 parallel dorsal and ventral margins, a cylindrical shell, tapering posterior- 

 ly, and the umbones near the anterior end. The surface is sometimes 

 irregularly sculptured. Sometimes nearly smooth, but always covered 

 with a strong yellowish to dark brown or blackish periostracum, which 

 never has hairy projections. The surface is often covered by a chalky in- 



