DIMYARIA. 159 



Ligula prcetenuis. — The Very Thin Ligula. Plate 

 XVnr. 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 striae. 

 One inch broad. Inhabits the British seas. 



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



Genus 54 — MACTRA — Linnceus. 



Generic Character. — Transverse, inequilateral, subtri- 

 gonal ; sides slightly gaping ; beaks prominent ; one pri- 

 mary 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 largo, the pri- 

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

 always erect. 



Mactra truncata. — The Truncated Mactra. Plate 

 XVIII. fig. 3. Strong, opaque, white; truncated on both 

 sides; mth a few concentric wrinkles. One inch and a 

 half long. Inhabits the British coasts. 



Genus 55 — MACTRINA Brown. 



Generic Character. — Subtriangular, inequilateral; umbo 

 prominent and obtuse ; left valve with a strong triangular, 

 slightly bifid tooth ; with triangular transverse pits on each 

 side, for the reception of two small depressed lateral teeth 

 in the opposite valve, with a triangular pit between them ; 

 ligament external. 



Mactrina triangularis. — The Triangular Mactrina. 

 Plate XVIII. 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. 



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