10 C. M. YONGE 



but there may be some rocking on the median fulcrum of the re- 

 duced hinge and hgament by reason of uneven contractions of the 

 two adductor muscles. The valves gape both anteriorly and pos- 

 teriorly. Judging by the greater erosion, most work may be done by 

 the umbonal (but also oldest) regions of the shell. The siphonal 

 extension of the boring is certainly enlarged by means of the four 

 periostracal scales. The reduced foot has no apparent function in 

 boring. 



ii. SuPERFAMiLY Adesmacea. All mcmbcrs of this large super- 

 family are borers, the more specialized Xylophaginidae and Tere- 

 dinidae living in wood, the majority of the Pholadidae boring into 

 stiff clay or rock. Species of Zirphaea which burrow into veiy heavy 

 clay probably indicate the manner in which the boring habit has 

 evolved. But these are now the only members of the superfamily 

 which do not bore. There is in all, including Zirphaea, great modifi- 

 cation of structure (see Lloyd, 1897; Purchon, 1955fl) in relation 

 to the purely mechanical process of boring. The valves tend to be 

 elongate, to bear rows of teeth or ridges, and to be very convex. 

 By a reduction of the mantle isthmus which runs between the two 

 lobes of the mantle, the ligament is either much reduced or com- 

 pletely lost, i.e. there is separation of the mantle lobes and shell 

 valves. Hinge teeth disappear and are replaced by dorsal articulat- 

 ing processes on which the shell valves, which gape anteriorly and 

 posteriorly, rock. In certain genera (e.g. Pholadidea, as shown in 

 Fig. 3, VA) there are secondary ventral articulatory surfaces. Both 

 adductors migrate dorsally so that they come to lie in the same line 

 as the dorsal articulating surface. In the anterior adductor this in- 

 volves a rolling upward of the anterodorsal region of the valves. 

 By cross fusion of the inner muscular lobe of the mantle margin, 

 a third, mid-ventral, adductor muscle is formed. Following the al- 

 ternate contraction of the anterior and posterior adductors, the 

 shell valves rock on the fulcrum of the dorsal articulation (and 

 ventral articulation where this is present ) . 



Attachment to the head of the boring, essential for effective bor- 

 ing action, is by way of the foot modified to form a sucker. This 

 projects forward through a wide anteroventral gape between the 



