MOLLUSCA. LAMELLIBRANCHIATA. 141 



Pectunculus. Shell thick, solid, sub-orbicular, equi- 

 valve, almost equilateral. Surface smooth or radiately 

 striated. Ligament external. Umbones central, slightly 

 curved towards each other, separated by a triangular 

 ligamental area provided with diverging striae. Hinge 

 with a semi-circular row of numerous, small, strong, 

 transverse teeth, obliterated at the centre in the older 

 forms by the growth of the ligamental area. Margins 

 crenulate inside, adductor impressions sub-equal. Pallial 

 line simple. Lower Cretaceous to present day. 



Nucula. Shell equivalve, trigonal, oval, closed, pos- 

 terior side very short ; umbones directed posteriorly. 

 Surface smooth or ornamented. Interior nacreous. Margin 

 of valves smooth or crenulated. Hinge-line angulated, 

 with a median internal triangular ligamental pit, on each 

 side of which is a row of numerous sharp teeth. Adductor 

 impressions sub-equal. Pallial line simple. Silurian to 

 present day. 



Leda Shell oblong, similar to Nucula, produced and 

 pointed posteriorly. Surface smooth or striated. Teeth 

 numerous. Ligamental pit internal. Pallial line slightly 

 sinuated. Margins smooth. Lunule lanceolate. Silurian 

 to present day. 



Trigonia. Shell thick, ornamented with tubercles or 

 with radiating or concentric ribs ; trigonal, very inequi- 

 lateral, anterior margin rounded, posterior produced and 

 angular. Generally with a ridge extending from the 

 umbones to the posterior border, cutting off an area which 

 has a different ornamentation. Umbones anterior, directed 

 posteriorly. Cardinal teeth diverging, striated, two in the 

 right valve, three in the left, the central tooth in the 

 latter being bifid. Ligament marginal, thick. Adductor 



