14 



r. M. YONGE 



B 



Fig. 5. Hiatella (Saxicava) sp.: A, showing full extension of siphons for 

 feeding and respiration; B, contraction of siphons (openings closed) ready 

 for boring, valves forced apart by water pressure, and thickened siphons grip- 

 ping sides of boring. (After Hunter, 1949.) 



modification from the protective reactions of non-boring animals." 

 Hiatella is confined to temperate seas in both the Atlantic and 

 the Pacific. It bores into comparatively soft rocks, i.e. muddy lime- 

 stones, mudstones, sandstones, calcite, and chalk (Hmiter, 1949). 

 Although all these rocks are broken down by acid, i.e. the binding 

 cement is acid soluble, there is no evidence that boring is aided 

 chemicallv. The form of the shell \aries so greatly, especialh' in 

 boring individuals, that specific identification is unusually difficult. 

 However, species of Hiatella are most efficient borers, cutting deep 

 borings which are round in cross section. 



V. SuPERFAMiLY Gastrochaenacea. Genus RoceUaria {Gastro- 

 chaena). Members of this genus are highlv specialized borers of 

 warm-temperate and tropical seas, always boring into calcareous 

 substrates. They are common in Indo-Pacific coral reefs (Otter, 

 1937). As shown in Fig. 6, there is some superficial resemblance to 

 the Pholadidae, notably in the similar possession of a suckerlike foot 

 projecting through a wide anteroventral gape and also of small 

 internal shell ridges to which the pedal muscles are attached 

 (Purchon, 1954). In other respects, however, there are major dif- 

 ferences, most obviouslv in the ligament, which is here very long, 

 so that the valves cannot possibly operate in boring as do those of 



