908 OYSTERS, AND ALL ABOUT THEM. 



(Margarita, Leach) ; and the restricted genus Avicula, 

 Lam., of which the Mytilus Hirundo of Linnaeus, a native 

 of the Mediterranean, and the Avicula macroptera, are 

 examples. 



To the genus Avicula Sowerby also refers the genus 

 Meleagrina of Lamarck ; regarding the distinctions between 

 them as of trivial importance. The shell in both is foli- 

 acious externally, and internally of a brilliant pearly lustre. 

 The left hand valve is contracted and notched posteriorly, 

 and so is the right, very slightly. Through the sinus thus 

 formed passes the byssus, by which the shells are moored 

 to rocks and stones. The ligamental surface is marginal 

 and broadest in the centre, and there is generally a small 

 tooth in each valve near the umbo. Generally this tooth 

 is most conspicuous in Avicula, but is not always found ; 

 whilst in Meleagrina it is, on the other hand, often appa- 

 rent, though sometimes absent. The muscular impression 

 is nearly central, somewhat circular and large. 



As thus characterized, Avicula will comprise two sec- 

 tions, namely, one in which the species have the hinge- 

 line considerable prolonged Avicula of Lamarck ; the 

 other comprising such species as are without that prolonga- 

 tion Meleagrina of Lamarck. 



Cuvier thus defines Avicula (Les Arondes) : The 

 shell has the valves equal, with a rectilinear hinge, and is 

 often carried out into wings ; the ligament is narrow and 

 elongated ; small dentilations often appear on the hinge, 

 on its anterior part ; and below the angle on the side near 

 the mouth is the notch for the byssus. The anterior 

 adductor muscle is still extremely minute. 



(a) Those species are termed Pintadines (Meleagrinae) 

 which have the auricles slightly salient. The term Avicula 

 (a) M. Deshayes states Avicula to be Monomyarian. 



