MOLLUSCA. LAMELLIBRANCHIA 223 



not preserved ; an elongate median tooth ; two adductor im- 

 pressions widely separated. The small (upper) valve generally 

 convex or conical, sometimes flat, with central umbo ; two straight, 

 elongate, grooved teeth ; the two adductor muscles were attached 

 to plates on either side of the teeth ; shell structure similar to 

 that of the larger valve, but with the external layer thinner. 

 Upper Cretaceous. Ex. R. angeiodes. 



Unicardium. Shell oval or rounded, inflated ; surface with 

 concentric lines or ridges. Umbones prominent, curved inwards. 

 In each valve a small cardinal tooth which is often obsolete, and a 

 posterior ridge separated from the margin by a furrow in which is 

 the external ligament. Adductor impressions elliptical. Trias to 

 Cretaceous. Ex. U. cardioides, Lias. 



Lucina (fig. 93 F). Shell orbicular or oval, slightly inequi- 

 lateral, usually ornamented with concentric lines or ridges. Lunule 

 usually distinct. An oblique furrow extends from the umbo to the 

 posterior border. Hinge usually with two cardinal and one or two 

 lateral teeth in each valve ; the lateral, or the cardinal, may be absent. 

 Ligament elongated, external, sometimes partly internal. Adductor 

 impressions well marked, the anterior elongated and placed mainly 

 within the pallial line, the posterior oval. Pallial line entire. 

 Margins of valves smooth or finely crenulated. Trias to present day. 

 Ex. L. borealis, Coralline Crag to present day. 



Cardium. Shell convex, slightly inequilateral, cordate or oval, 

 generally closed. Umbones prominent, incurved, turned slightly to 

 the anterior end. Surface with radiating ribs, which are often 

 spiny. Margins of valves crenulated. Right valve with one or two 

 cardinal teeth, two anterior laterals, and one or two posterior 

 laterals; left valve with two cardinals, one anterior lateral and 

 one posterior lateral. Ligament external. Adductor impressions 

 shallow. Pallial line entire. Trias to present day. Ex. C. acu- 

 leatum, Pleistocene and Recent ; C. edule, Pliocene to present 

 day. 



Protocardia. Similar to Cardium^ but with radiating ribs 

 on the posterior part of the shell only, the remainder with 

 concentric ribs. Jurassic to present day. Ex. P. hillana, Upper 

 Greensand. 



