144 MOLLUSCA. LAMELLIBRANCHIATA. 



bones small, close together. Lunule distinct. Ligament 

 internal. Hinge with two cardinal teeth, and one posterior- 

 lateral and sometimes one anterior-lateral. Adductor 

 impressions deep; pedal impressions small, distinct. Pallial 

 line simple. Lower Cretaceous to present day. 



Chama. Shell irregular, thick, inequivalve, fixed by 

 the umbone of the larger valve (generally the left, some- 

 times the right). Umbones spiral or subspiral, that of 

 the fixed valve longer than the other. Surface with 

 concentric lamella or spines. The fixed valve larger and 

 deeper than the other. In each valve a strong cardinal 

 tooth, and sometimes in the inferior valve a narrow 

 curved posterior tooth also. Ligament external, in a deep 

 groove, prolonged towards the umbones. Adductor im- 

 pressions large, often rough. Pallial line simple. Cre- 

 taceous to present day. 



Hippurites. Shell large, massive, conical or sub- 

 cylindrical, very inequivalve, fixed by the apex of the 

 larger valve. The larger (right) valve, conical, striated or 

 smooth, and with three parallel furrows on the cardinal 

 side, and three corresponding folds in the interior. Smaller 

 (left) valve flattened or slightly convex, operculiform, 

 porous, the pores leading into canals; with a central 

 umbone, two large cardinal teeth ; two large adductor 

 impressions ; pallial line simple. The smaller valve is 

 formed of two layers, the outer is thin and prismatic, the 

 inner is porcellanous and traversed by numerous canals. 

 The larger valve is also composed of two layers, the outer 

 is formed of small prisms arranged in parallel layers 

 obliquely to the surface of the shell, the inner is porcel- 

 lanous, and formed of thin leaflets. Middle and Upper 

 Cretaceous. 



