12 c. M. yongf: 



strates including slate, shales, sandstone, chalk, marl, and even 

 peat and submerged wood. Pholadidea penita has been found in 

 concrete casings. In all, the fused siphons are long, the basal half, 

 which is encased in a protective periostracum, representing pos- 

 terior extension of the mantle cavity and housing the posterior 

 region of the gills. The pholads do not appear to bore significantly 

 into limestone and, although usually the most important rock borers 

 in other regions, are certainly largely absent from coral reefs. 



Epifaunal Origin 



iii, SuPERFAMiLY Veneracea. Family Petricolidae. The typi- 

 cal venerid burrows very superficially, but in certain species byssal 

 attachment persists, and these forms inhabit rock crevices. In the 

 Petricolidae this habit is taken further and the animals bore into 

 the rock. There is an interesting range of specialization within the 

 genus Petricola. Thus in P. carditoides (Fig. 4A), common in holes 

 in rocks on the California coast, the animal does no more than en- 

 large these as it grows; the process is entirely mechanical by means 

 of the shell valves, water pressure being possibly materially assisted 

 by the opening thrust of the long and unmodified ligament. The 

 byssus is not retained in boring. Great secretion of mucus, both in 

 the mantle cavity and in the siphonal embayment, is associated with 

 this mode of life (Yonge, 1958). The shell may be almost globular 

 or elongate; it is often very irregular in form. At the other extreme 

 comes P. pholadiformis (Fig. 4B) — so named because of the super- 

 ficial resemblance of its elongate, ridged shell to that of a pholad — 

 which bores with great efficiency (Purchon, 1955/?). Species of 

 Petricola bore into a variety of substrates, from stiff mud, mudstone, 

 and shales to limestone. Petricola lapicida, with boring capacities 

 intermediate between those of P. carditoides and P. pholadiformis, 

 is not uncommon in beach rock, coral boulders, etc., on the Great 

 Barrier Reef ( Otter, 1937 ) . 



iv. SuPERFAMiLY Saxicavacea. Genus HiatclUi ( Saxicava ) . Spe- 

 cies of this widely distributed genus may either live in rock crevices 

 attached by byssus threads or bore. It is not clear whether certain 

 species always bore and others always "nestle"; indeed, there is 



