ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Genus ADULA H. and A. Adams 1857 



Adula H. and A. Adams, Genera of Recent Mollusca, vol. 2, 1857, p. 



517. 

 Type of genus: Mytilus soleniforrnis Orhigny 1846 (monotypy). 

 Remarks: Adula was described as a subgenus of Perna Adanson = 

 Modiolus Lamarck with the following diagnosis: "Shell elongate, cylin- 

 drical, posterior side obliquely truncate, beaks sub-central." Only the 

 type species was included in the new subgenus. 



Although the four species here referred to Adula, which in my opinion 

 constitutes a good generic unit, are rather different, they have several 

 characters in common which serve to separate them from other mytilid 

 genera. The elongate form simulates the species of Lithophaga and ap- 

 parently the species are generally borers, too. They have the anterior 

 byssal retractors fastened before the umbones, and the siphonal pro- 

 longations of the posterior part of the mantle are built like those found 

 in Lithophaga. The sculpture of some species and the filthy covering of 

 the posterodorsal triangle are also characters which make them look like 

 a Lithophaga, and the gonads are extended into the mantle just as in 

 that genus. There are, however, other characters typical for these spe- 

 cies only. The posterior adductor, which is placed above the middle of 

 the valve, is confluent with the posterior retractor muscles lying mainly 

 in front of the adductor. Below the adductor are seen rather distinct 

 scars of the retractors for the siphonal mantle prolongations. The um- 

 bones are placed farther back than in Lithophaga, and the anterodorsal 

 margin is thickened. The crenulations on the margins vary considerably. 

 Three species have distinct crenulations on the anterior margins, two 

 have them also behind the ligament, while one species has completely 

 smooth margins. Radiating sculpture on the lunule is found in two 

 species, but not in the others. 



Dall (1921) and other authors have considered Adula to be a sub- 

 genus of Botula Morch ; but they are very different and apparently not 

 closely related. The strongly curved umbones and the peculiar place where 

 the anterior retractor is fastened on the anterior thickened margin itself, 

 make the species of Botula look very different from those of Adula. 



The differences in the conchological characters of the four species here 

 referred to Adula clearly show how dangerous it is to rely on one char- 

 acter only in separating mytilid genera. It would have been diflicult to 

 place dicgensis in this genus without seeing the typical form of the pallial 

 siphonal elongations and the same scar of the muscles as was observed in 



