1IOLLTJSCA. 



157 



scallop (Pecten maximus). Freshwater mussels are various species 

 of Unio and Anodonta. 



Anomiidce. 

 Anomia. 

 Placunanomia. 



Placunida. 

 Placuna = Placenta. 



Ostreidce. 



Gryphaea. 

 Ostrea (Oyster). 



Spondylida. 

 Spondylus. 



Pectinidce. 



Amusium =Pleuro- 



nectia. 

 Pecten (Scallop). 



Limida. 

 Lima. 



Aviculidce. 



Malleus. 



Vulsella. 



Perna. 



Meleagrina. 



Avicula. 



Pinna. 



Arcidce. 

 Leda. 



Nucula. 



Axinaea = Pectun- 



culus. 

 Cucullaea. 

 Anomalocardia. 

 Area. 



PinnidcB. Trigonia. 



Prasina. 

 Dreissena. 

 Mytilus (Mussel). 

 Modiola. 

 Lithodomus=Litho- Unio. 

 phagus.. 



Unionidce. 



.^Etheria. 



Hyria. 



Mycetopus. 



Anodonta. 



Margaritana. 



Order II. SIPHONIATA. 

 MACROTRACHIA. 



Headless mollusks with one or two respiratory siphons. Lobes 

 of the mantle more or less united. 



Locomotion, effected by the more developed foot, is general in 

 this order, and the respiratory system is more complex. For this 

 purpose there are one or two tubular prolongations [siphons] 

 from the margin of the mantle ; when two. generally united the 

 one inhalant, the other exhalant, and furnished with muscular 

 fibres for their emission and retraction, but in some cases the 

 siphon is incapable of being retracted. The foot is sometimes 

 provided with an orifice through which the water enters, and 

 when swollen by its admission the foot may exceed the capacity 

 of the shell. 



This order includes the clam [of North America] (Mactra 

 soUdissima) [the river-clams are Unios], cockle (Cardium cdule], 

 cob (My a arenaria), razor-shell (Solon siliqua), and ship- worm 

 ( Teredo navalis). The shell of Tridacna gigas, one valve of which 



