94 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



triangle is bounded by distinct dorsal and ventral furrows and is fur- 

 nished with a bluish-white incrustation with vertical wrinkles, especially 

 in the median part. Periostracum yellowish to yellow-brown, covered 

 with a loose, granulated incrustation. Inside margaritaceous, posterior 

 adductor relatively large and near posterior margin. Ligament shorter 

 than half the length of the valve. 



According to Carpenter, the type specimen measured 0.36 inches in 

 length, 0.14 in height, and 0.15 in diameter. The two specimens from 

 the Mexican islands in this material measure 15 mm by 7 mm and 14.2 

 mm by 6 mm, excluding the incrustation. The small Galapagos specimen 

 has a length of 6 mm. 



Occurrence: Apparently this species has not been obtained by collectors 

 since Reigen did his collecting more than a hundred years ago and picked 

 up the type specimen. It was therefore very interesting to find three ex- 

 amples of this apparently very rare species in the Hancock Collections. 

 The three specimens were obtained on Isla Socorro and Isla Clarion on 

 the shore, and at Isla San Cristobal (Chatham Island), Galapagos Is- 

 lands, in a depth of 32 fms. The records are too few to allow a discus- 

 sion of the occurrence of this species, but they seem to indicate an unusual 

 type of distribution. 

 Distribution: Mazatlan, Mexico, to the Galapagos Islands. 



Subgenus DIBERUS Dall 1898 



Diberus Dall, Trans. Wagner Free Inst. Sci., vol. 3, 1898, p. 799. 

 Type of subgenus: Modiola plumule Hanley 1843. 



Remarks: Dall (1898) says: "Resembling Myoforceps, but with two or 

 more radial sulci extending backward from the beaks, with the incrusta- 

 tion plume like, arranged in a distinct pattern on the areas between the 

 sulci, and, when projecting beyond the ends of the valves, apposited sym- 

 metrically, not alternate and twisted as in the last section." 



The most conspicuous character separating species of Diberus from 

 the other species of Lithophaga is the plumelike incrustation on the upper 

 and posterior part of the valves. The anterior byssus retractor is fan- 

 shaped where it is fastened to the valves and makes an elongate narrow 

 scar from just behind the umbo halfway down the anterior slope. The 

 posterior retractor scar is, at least in the west American species, placed 

 above the posterior adductor and separated from it. 



Iredale (1939) has named two sections from Australia, which seem 

 to be identical with, or very close to, Diberus, viz., Exodiberus, type 

 Lithophaga calcifer Iredale, and Salebrolabis, type Lithophaga divaricalx 

 Iredale. 



