140 DhMYAIRA. 



in each valve, projecting inwards, furnished with a pit for tlie 

 reception of the cartilage, and in some species a minute tooth. 



Ligula prcBteyiuis.— The Very Thin Ligula. Plate XVIII. 

 fig. 2. Oval, flat, thin, brittle ; a little gaping ; valves with a 

 single, spoon-like tooth in each, projecting horizontally inwards ; 

 white, with a few concentric stria. One inch broad. Inhabits 

 the British seas. 



*** Ligament internal ; shell gaping at the sides. 



Genus 5i.~MA CTR A.— Linnaus. 



Generic Character. — Transverse, inequilateral, subtrigonal ; 

 sides slightly gaping; beaks prominent ; one primary compressed 

 tooth in each valve, and an adjacent heart-shaped cavity; two 

 lateral compressed teeth, situated near the hinge, and inserted ; 

 ligament internal and placed in the pit of the hinge. 



In some species where the cavity of the hinge is very large, the primary 

 tooth is oblique, and always obsolete ; the lateral teeth, however, always 

 erect. 



Mactra truncata. — The Truncated Mactra. Plate XVIII. 

 %. 3. Strong, opaque, white ; truncated on both sides ; with 

 a few concentric wrinkles. One inch and a half long. Inhabits 

 the British coasts. 



Genus 55.~MACTRIlSlA.—Brown. 



Generic Character. — Subtriangular, inequilateral ; umbo pro- 

 minent and obtuse ; left valve with a strong triangular, slightly 

 bifid tooth ; with triangular transverse pits on each side, fpr 

 the reception of two small depressed lateral teeth in the oppo- 

 site valve, with a triangular pit between them ; ligan?ent ex- 

 ternal. 



Mactrina triangularis — The Triangular Mactrina. Plate 

 X VIII. fig. 30. Strong, opaque, white ; inside white, not very 

 glossy ; margin strongly crenated. Length, one eighth of an 

 inch. Inhabits the sea on the Dorsetshire and Kentish coasts. 



Genus 56.— LUTRA RIA Lamarck. 



Generic C/iamc/er.— Inequilateral, transversely oblong, or 



