92 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



crustatlon, generally harder and thicker on the posterodorsal triangle and 

 usually projecting beyond the posterior margin of the valve. The margins 

 are always smooth and there are no toothlike crenulations in the hinge. 

 The lunule is not defined. 



The mantle margins are smooth except for the posterior part, which 

 in preserved specimens is folded. The inner folds of the mantle, project- 

 ing posteriorly, form siphons, of which the ventral one is open below as 

 is usual in mytilids. There are no visible scars formed by the siphonal 

 musculature. The anterior adductor is elongate and rather large, placed 

 near the margin; the posterior adductor is small and roundish. The 

 anterior retractor of the byssus is fastened before the umbones and 

 separated from the adductor; the posterior retractor es pedis et bysii are 

 continuous, small, and weak, and usually fastened above the posterior 

 adductor. The byssal opening is behind the foot, large and with a func- 

 tional gland, even in large specimens. The foot is small and pointed, with 

 a ventral median furrow. The gonads are found in the mantle on both 

 sides. All species bore in loose rock, corals, or the shells of other mollusks. 



The genus Lithophaga has been divided into several "sections," mainly 

 by Dall and Iredale, because of the differences in the chalky incrustation 

 of the valves. These "sections" cannot be given the same value as the 

 other groups of mytilids used in this paper, since there is a great deal of 

 variation even among specimens of one population. It is sometimes very 

 difficult to assign a specimen to a "section" on the basis of the characters 

 given in the diagnosis alone. However, a few groups are useful in the 

 classification, even if they generally do not indicate groups of species with 

 different geographic distribution or those which seem to be genetically 

 related. Small specimens or specimens with the incrustation cleaned off, 

 are nearly impossible to classify. 

 Key to subgenera : 



1. Valves without a calcareous incrustation (Lithophaga s. s.) 

 Valves with a calcareous incrustation 2 



2. Incrustation seen from behind oval and closed and 

 with a vertical internal tube, which has one dorsal 



and one ventral opening Stumpiella n. subg. 



Incrustation otherwise, always without dorsal and 

 ventral openings 3 



3. The thickened part of the incrustation with ir- 

 regular wrinkles more or less like a feather or 

 with strong radiating grooves, and projecting dis- 

 tinctly beyond the valve Diberus Dall 



