78 AXEL A. OLSSON 



of a nearly black or purple-black color; it is unequally divided into two 

 sections by an incised line starting at the beak and running obliquely down- 

 ward and in an anterior direction to the hinge line (divisional ligamental 

 line); on the posterior side of this hne, the cardinal surface is striated hori- 

 zontally and smooth, while on its longer anterior side it is crossed by a 

 series (often 14 or more) of evenly spaced oblique slits to which threads of 

 the tensilium are attached. The hinge line is long, narrow and straight, its 

 teeth small and numerous, vertical or inclined, divided into an anterior 

 and posterior set by the incised line mentioned above; in an average speci- 

 men, there are 28 small teeth in the posterior set and about 75 in the 

 anterior set. The sculpture is nearly always destroyed by wear, especially 

 on the umbones and across the mid-surface of the valves but small patches 

 usually remain along the ventral side and on the posterior slope; in the 

 spots where the sculpture is still retained, it is seen to consist of fine, radial 

 riblets crossed by crowded concentric layers producing a pattern of lacelike, 

 scalloped teeth, their interspaces retaining small tufts of a straw-colored 

 periostracum. Interior of shell porcellaneous, the pallial line lying quite 

 close to the ventral margin. Adductor scars distinct, the ventral margin 

 plain. 



This is a curious genus, represented by a single known species (Litharca 

 lithodomus) ranging from Ecuador northward to Panama and Costa Rica. 

 It was assigned by Frizzell to a separate subfamily the "Litharcinae" but 

 anatomical observations made possible by the discovery of living specimens, 

 show that the genus is closely allied to Area sensu stricto. The elongated 

 form of the shell is merely an adaption to a rock-boring habit in which the 

 anterior end of the shell and not the posterior became drawn out and 

 lengthened. 



The longer side of the shell is anterior and assumes a flattened, wedge- 

 like form at its end; the much shorter and carinate side is posterior and is 

 often more or less covered with extraneous marine growth. The beaks and 

 umbones are always worn but commencing at a point below the assumed 

 position of the beak, a faintly engraved line (divisional ligament line of 

 Frizzell) passes obliquely across the cardinal area to the hinge line, dividing 

 the area and the line of the hinge teeth into two unequal sections. The dip 

 of this line is usually about 30 degrees or somewhat less depending upon 

 the size of the specimen in hand. In the larger of the specimens figured 

 (length 93 mm.), the hinge has 27 teeth in the posterior set and about 94 

 in the anterior set. The lengthening of the shell of Litharca, so as to ac- 

 commodate itself to the narrow confines of a straight bore, was accomplished 

 by a flattening of the umbones and a pulling out or lengthening of the axis 

 of the shell mainly along the anterior side. As a consequence of this down- 

 ward squeeze, the umbones and the beaks were pushed backwards thus 

 causing the ligamental line to slant down sharply, its inclination lowering 

 as the elongation of the shell increased. Our smallest specimen has a length 

 of 13 mm. This shell has 12 hinge teeth in the posterior set and 18 in the 

 anterior. The umbone is already so deeply worn that the shape of the beak 

 has been destroyed. 



Litharca Uthodomus (Sowerby) Plate 5, figures 1, la-It 



Byifarca lithodomus Sowerby, 1833, Proc. Zool. Soc. London, p. 16. 



