MOLLUSCA. LAMELLIBRANCHIA 211 



Nuculana ( = Leda). Similar to Nucula. Posteriorly the 

 shell is produced and pointed, and provided with a ridge or carina. 

 Pallial line with a small sinus. Margins smooth. Lunule lanceolate. 

 Silurian to present day. Ex. N. lachryma, Inferior Oolite to Corn- 

 brash ; N. caudata, Pliocene to present day. 



Ctenodonta. Shell oval or elongated, nearly equilateral, 

 smooth or with concentric striee. Ligament external. No area. 

 Hinge curved or angular, with numerous small teeth. No internal 

 ligament-pit. Pallial line simple. Cambrian to Carboniferous. 

 Ex. C. pectunculoides, Ordovician. 



Area. Shell thick, generally equivalve, sub-quadrangular, 

 ventricose, ornamented with radiating ribs and concentric strise ; 

 margins smooth or dentate ; closed or gaping ventrally. Hinge 

 straight, with numerous, small, equal, transverse teeth. Umbones 

 prominent, separated by the large areas, which have numerous 

 ligamental grooves converging from the hinge-margins to the um- 

 bones. Adductor impressions sub-equal, the anterior rounded, the 

 posterior divided. Pallial line simple. Jurassic to present day. 

 Ex. A. tetragona, Pliocene to present day ; A. granosa, Eecent. 



Cucullaea. Shell similar to Area. Hinge with short central 

 transverse teeth, and two to five lateral teeth nearly parallel to the 

 hinge-margin. Posterior adductor fixed to a thin raised plate. 

 Jurassic to present day. Ex. C. fibrosa, Upper Greensand. 



Pectunculus ( = Glycimeris, Axincea). Shell thick, solid, sub- 

 orbicular, equivalve, almost equilateral. Surface smooth or radially 

 striated. Ligament external. Umbones central, slightly curved 

 posteriorly, separated by a small triangular area provided with 

 diverging grooves for the ligament. Hinge arched or semicircular, 

 with a row of numerous, small, strong, transverse teeth, obliterated 

 at the centre in the older forms by the growth of the area. Margins 

 crenulate inside ; adductor impressions sub-equal — the anterior sub- 

 triangular, the posterior oval or rounded. Pallial line with a very 

 small sinus. Cretaceous to present day. Ex. P. glycimeris, 

 Pliocene to present day. 



2. Hinge dysodont, but teeth sometimes rudimentary 

 or absent. Anterior adductor smaller than the posterior, 

 sometimes absent in the adult. Pallial line simple. 



14—2 



