56 



9 ( 8). Height of shell slightly less than, or equal to length. Hinge system 

 with two equal teeth on left valve and two on right, one of which 



is longer and central in position Pholadidae, Figure 41. 



Shell different in shape, not round. 



All margins of shell round and merge smoothly into one another. 



12(13). Shell transversely oval. Anterior margin ventrally produced. Ante- 

 rior shoulder much longer than posterior. Anterior part of hinge line 

 with special large tooth. Ocular spots absent . .Tellinidae, Figure 42. 



13(12). Shell longitudinally oval, ovate. Anterior margin not produced ven- 

 trally. Shoulders differ only slightly in length. Special teeth absent. 

 Ocular spots present. 



14(15). Anterior margin blunt. Ventral margin broad, slightly rounded, par- 

 allel to hinge line. Relief of concentric lines quite distinct 



Lithophagidae, Figure 43. 



10(5). 

 11(18). 



Figure 36: Structure of veliconcha shells in different families. 



1 — Pectinidae; 2 — Mytilidae; 3 — Ostreidae, Cordiidae; 4 — Solenidae; 5 — Veneridae; 



6 — Tellinidae; 7 — Mactridae; 8 — Myidae; 9 — Clinocardiidae; 10 — Kelliidae; 11 — 



Hiatellidae; 12 — Teredinidae; 13 — Pholadidae; pi — pallial line. 



