MONOMYARIA. 133 



base acute, affixed by abyssus; beaks acute, nearly straight, 

 and terminal ; hinge lateral, generally with teeth ; ligament 

 marginal, and partly internal; muscular impression elon- 

 gated, clavate, and sublateral. 

 Section * Longitudinally fuirowed. *» Without fui-rows. 



Mytilus edulis — The Edible Mytilus. 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.* 



Distinguished from the genus Mytilus by the beaks not being tenni- 

 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 IIL TRIDACNITES. 



Transverse, equivalve, the muscular impressions situated 

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

 side. 



* Both in Mytilus and Modiola there are in reality two muscular 

 impressions, although one is very smalL 



M 



