498 LAMELLIBRANCHIATA. 



long cylindrical tubular projection or beak. Posteriorly the 

 shell is elongated and contracted, the valves being widely 

 deficient or gaping at the extremity. The hinge-line is long 

 and straight ; and two cardinal teeth, with a hinder lateral 

 one, appear to be present. (It should be mentioned that 

 this description is sometimes reversed, the truncated side 

 being regarded as posterior.) 



FAM. 11. LUCINIDJE. Shell orbicular, free; cardinal teeth 

 1 or 2 ; lateral teeth 1-1, or obsolete. Mantle-lobes open 

 below, with one or two siphonal orifices behind ; foot elon- 

 gated, cylindrical, or strap-shaped. Taken as a whole, the 

 family is principally Secondary, Tertiary, and Eecent, its 

 Palaeozoic representatives being mostly imperfectly under- 

 stood, and referred here with doubt. In Lucina itself, the 

 type of the family (fig. 364, c), the shell is rounded, with a 



Fig. 364. A, Interior of the right valve of CorUs pectuncuhis Eocene ; B, Interior of the 

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

 Jurassic. 



lunule beneath the beak ; the ligament is in a deep groove, 

 nearly internal ; and the teeth have the typical arrangement 

 of the entire group, though some are occasionally obsolete. 

 Little can be said with certainty as to the Palseozoic shells 

 usually referred to Lucina, but the genus is abundantly rep- 

 resented in Secondary and Tertiary deposits. Corbis (fig. 

 364, A), with many species from the Jurassic onwards, is 

 very like Lucina, but has the surface concentrically fur- 

 rowed, with denticulate edges. Diplodonta (Cretaceous to 

 Eecent) has two cardinal teeth in each valve, the anterior 

 in the right and the posterior in the left being bifid. Kellia, 



