NO. 1 



SOOT-RYEN : THE FAMILY MYTILIDAE 



89 



the other species. These characters alone separate diegensis from the 

 Modiolus group. A. diegensis is free-living, and therefore somewhat dif- 

 ferent from the other species, but the free-living habit alone is not suffi- 

 cient to separate a species from its relatives in a genus. 



Species of the genus Adula are distributed from Peru to British 

 Columbia, and Japan. The genus is related to Lithophaga Roding, Zeli- 

 thophaga Finlay, Terua Dall, Bartsch, and Rehder, and perhaps to 

 Botulopa Iredale. 

 Key to the west American species: 



1. Shell with irregular vertical or wrinkled 

 sculpture, margins without crenulations 

 Shell smooth or with radiating sculpture 



2. Radiating sculpture on the anterior part 

 and before the keel, margins crenulated 

 anteriorly and strongly so behind the 

 ligament 



Shell smooth or with few radiating 

 striae anteriorly, posteriorly curved, 

 with a filthy incrustation 



3. Shell elongate, not higher posteriorly 

 than anteriorly, lunule generally with- 

 out radiating lines 



Shell shorter, distinctly highest pos- 

 teriorly, lunule generally with distinct 

 radiating lines, which form internal 

 ribs diegensis (Dall) 



falcata (Gould) 



2 



soleniformis (Orbigny) 



calif ornianus ( Philippi ) 



Adula falcata (Gould) 1851 

 Plate 9, fig. 49 



Lithodomus falcatus Gould, Proc. Boston Soc. Nat. Hist., vol. 4, 1851, 



p. 92. 

 Holotype: ? 



Type loc: Monterey, California. 



Remarks: This species has the umbones near the anterior end. The 

 peculiar sculpture of vertical or oblique or partly irregular wrinkles is 

 typical for the species. The posterodorsal triangle has a filthy incrusta- 

 tion. The scars of the siphonal retractors consist of many small impres- 

 sions just below the posterior adductor. The ventral siphon has irregular 



