86 GENUS XVI. MYTILUS. 



The liinges of tlie Muscles are very simple, being generally 

 nothing- more than an external cartilage fixed to the edge of 

 each valve ; sometimes there are a few small teeth on the edge, 

 but oftener a longitudinal ridge just withinside. The shells 

 are said to be longitudinally striate when marked with lines 

 from the beak to the rounded end, and transversely striate 

 when marked with lines across those. 



LinnsBus has described in the first division 3 species, in the 

 second 2, and in the third 15 : in all, 20 ; Gmelin has added 

 to the third division 38; Dr. Turton has added to the third di- 

 vision 6. Total, 64. 



Lamarck places some of the species with the Oysters, and 

 divides the rest into the five following genera : 

 Mytilus. Shell longitudinal, terminated by a straight beak, 

 lengthened to a point, fixing itself by a byssus ; hinge 

 generally without teeth ; only one muscular impression 

 (see Mytilus edulis, the common Muscle). 

 MoDioLj. Shell subtransverse, the posterior side very short, 

 with the beaks turning toward the short side ; the hinge 

 withovit any teeth ; only one muscular impression (see 

 Mytilus Modiolus, f. 48). 

 Anodonta. Shell transverse, having three muscular impres- 

 sions ; hinge without any teeth (see Mytilus anatinus, 

 /. 49). 

 A vicuLA. Shell free, valves unequal, a little gaping near the 

 beaks, fixing itself by a byssus ; the liinge without teeth, a 

 little swelling ; the hollow for the cartilage oblong, mar- 



