228 MOLLUSCA. LAMELLIBKANCHIA 



bones turned a little to the posterior side. Right valve larger than 

 the left. External ligament short, prominent. Hinge thickened 

 behind the umbo forming a stout process or ossicle in each valve to 

 which the ligament is fixed. Adductor impressions small. Pallial 

 sinus not deep. Trias to present day. Ex. T. pubescens, Coralline 

 Crag to present day. 



7. Shell thin. Hinge either without teeth or with 

 only imperfectly developed teeth, and without hinge- 

 plate. Ligament external. Two nearly equal adductors. 

 Pallial line simple. This group is a provisional one, and 

 includes primitive Palaeozoic genera, the affinities of 

 which have not yet been determined. 



Gram my si a. Shell elongate-ovate, very inequilateral, orna- 

 mented with concentric furrows, and one or more radial folds passing 

 from the umbo to the postero-ventral border. Um bones placed 

 anteriorly. Lunule very deep. Hinge-margin thick, without teeth. 

 Anterior adductor very small, posterior large. Pallial line simple. 

 Silurian and Devonian. Ex. G. cingulata, Silurian ; G. hamilton- 

 ensis, Devonian. 



Cardiola. Shell thin, convex, oval, generally inequilateral ; 

 umbones prominent, incurved. Surface with well-marked radiating 

 and concentric grooves. Hinge-line straight, probably with very 

 small teeth ; ligamental area large, horizontally grooved. Muscular 

 impressions unknown. Silurian and Devonian. Ex. C. interrujpta, 

 Lower Ludlow, etc. 



Cardiomorpha. Shell thin, smooth or with concentric 

 lines ; sub-quadrate or rounded, inequilateral, very convex. Umbones 

 prominent, curved forwards ; no lunule. Hinge toothless. Ex- 

 ternal ligament small. Adductor impressions shallow ; pallial line 

 simple. Principally Carboniferous. Ex. C. oblonga, Carboniferous 

 Limestone. 



Edmondia. Shell sub-quadrate or ovate, convex, inequi- 

 lateral ; surface with concentric lines or ridges. Umbones anterior ; 

 no lunule, no escutcheon. Hinge toothless, with a thick ridge 

 posterior to the umbones and separated from the edge of the valve 



