MONOMYARIA. 133 



base acute, affixed by a byssus ; beaks acute, nearly straight, 

 and terminal; hinge lateral, generally with teeth; ligament 

 marginal, and partly internal; muscular impression elon- 

 gated, clavate, and sublateral. 

 Sectioa * Longitudinally furrowed. ** Without furi'ows. 



Mijtilus edulis. — The Edible MrxiLUs. Plate XV. fig. 

 16. Oblong-oval, nearly smooth, pointed, and slightly cari- 

 nated at the beak, truncated on one side, and dilated on the 

 other; covered with an olivaceous epidermis; beneath which 

 it is radiated with blue or purple ; internal margin blue, the 

 concave part white. Two to three inches long. Inhabits 

 the British seas. 



Genus 29 MODIOLA — Lamarck. 



Generic Character. — Subtransverse, equivalve, regular; 



the posterior side very short; beaks nearly lateral, depressed 



on the short side; hinge without teeth, lateral and linear; 



cardinal ligament placed in a marginal furrow, and nearly 



internal; one muscular impression in each valve, which is 



sublateral and elongated.* 



Distingmshed from the genus Mytilus by the beaks not being termi- 

 nal but placed a little below the apex. 



Modiola discors. — The Discordant Modiola. Plate 

 XV. fig. 17- Oblong-oval, very convex, produced and nar- 

 rower at the anterior end; longitudinally striated on both 

 sides, and transversely in the middle; outside green; inside 

 white or pale pink, and somewhat pearly; margin crenulated 

 at the base ; beak oblique. One half inch long. Inhabits 

 the British seas. 



TRIBE III. — TRIDACNITES. 



Transverse, equivalve, the muscular impressions situated 

 under the centre of the lower margins, and extended on each 

 side. 



» Roth in Mytilus and Modiola there are in reality two nuiseular 

 impressions, although one is very small, 



M 



