746 



LAMELLIBRANCHIATA. 



process " (Woodward). Species of Teredo occasionally reach a very 

 large size, and they are known in the fossil state both by their shells 

 and by their burrows in wood. The genus seems to have com- 

 menced in the Lias, and is well represented at the present day. 

 Numerous Tertiary species are known, but the recognition of the 

 existence of the " Ship-worms " in past time very generally depends 

 simply upon the presence of their filled-up burrows in fossil wood. 



Order XII. Lucinacea. 



This order includes marine Lamellibranchs, with typically but a 

 single branchia on each side and with two adductor muscles, the 

 mantle-lobes being more or less free, and the foot usually vermiform. 

 The shell is free, non-nacreous, the hinge with cardinal and lateral 

 teeth, and the pallial line entire. The only family included in this 

 order is that of the Lucinidcs, the precise limits of which, as regards 

 fossil forms more particularly, are still uncertain. 



Family i. Lucinid^:. — The mantle-lobes in this family are open 

 below, with one or two siphonal apertures behind, and the foot is 

 elongated, cylindrical, or strap-shaped. The shell is orbicular and 

 free, with one or two cardinal teeth, and generally a single lateral 

 tooth on each side. The ligament is partially or wholly internal, or 



Fig. 632. — A, Interior of the right valve of Cordis pectunculus — Eocene ; B, Interior of the 

 right valve of Diplodonta lupinus— Miocene; c, Interior of the left valve of Lucina striatula— 

 Jurassic. 



in some cases (Diplodonta) external. The anterior adductor impres- 

 sion is usually elongated. Taken as a whole, the family is principally 

 Secondary, Tertiary, and Recent, its Palaeozoic representatives being 

 mostly imperfectly understood, and referred here with doubt. In 

 Lucina itself, the type of the family (fig. 632, c), the shell is 

 rounded, with a lunule beneath the beak ; the ligament is in a deep 

 groove, sometimes nearly or quite internal ; and the teeth have the 

 typical arrangement of the entire group, though some are occasion- 



