KOCK-BORING ORGANISMS 9 



of rock-boring bivalves varies very greatlv, as brieflv outlined 

 below. 



Infatinal Origin 



i. SuPERFAMiLY Myacea, Plalijodon cancellatiis, the sole boring 

 species (and genus), somewhat resembles the soft-shelled clam, 

 Mija arenaria, to which it is closely related. It is confined to the 

 California coast between 33° and 38° north latitude, where it oc- 

 curs between tidemarks, boring in relatively soft mudstones (Yonge, 

 1951), It bores mechanically by means of the shell valves, which 

 are ridged and much eroded in the umbonal regions. Compared 

 with Mija, the valves are more convex (see Fig. 2), and the con- 



A B 



Fig. 2. Platyodon cancellatus. A, animal in boring, showing erosion of shell 

 around umbones, also periostracal "scales" near tip of siphons; B, section 

 through boring in umbonal region. (After Yonge, 1951.) 



densed ligament or chondrophore is displaced somewhat posteriorly. 

 The horny periostracum which covers the long siphons is thickened 

 near the tip to form four "scales," which represent the only obvious 

 adaptation for boring. 



Platyodon bores directly into the rock, i.e. without twisting. As 

 a result the boring is not rounded in cross section, but, in the region 

 occupied by the shell, has "dorsal" and "ventral" ridges, the former, 

 which lies between the rounded umbones, being very pronounced 

 ( Fig. 2B ) . Boring is the result of lateral pressure by the valves due 

 to taking of water into the mantle cavity, followed by closure and 

 withdrawal of the siphons. Such pressure can be only slightly in- 

 creased by the opening thrust of the somewhat reduced ligament. 



