THE LAMELLIBRANCHIA 257 
provided with a highly developed byssogenous apparatus. ~ The 
order comprises five sub-orders—the Anomiacea, Arcacea, Mytilacea, 
Pectinacea, Dimyacea. 
Sus-OrRpDER 1. ANOMIACEA. 
Very asymmetrical animals with a single large posterior adductor 
muscle. The heart is not contained in the pericardium, lies dorsad of the 
A portion of the gill of Mytilus, showing the filaments ; the median part is cut out. a.b.v, 
afferent branchial vessel ; ¢.b.v, efferent branchial vessel ; 7./j, interfilamentar ciliated junction ; 
i.1.j, interlamellar connective tissue junction. (After Bonnet.) 
rectum, projects into the pallial cavity, and gives off a single and anterior 
aorta. The reflected borders of the inner gill-plates of either side are 
fused together in the middle line. The gonads open into the kidneys, 
and the right gonad extends into the mantle. The shell is thin and the 
animal fixed, 
Family ANomimpAgr, Adams. Foot small. The inferior (right) valve 
of the adult is perforated to admit of the passage of the byssus. Genera 
—Anomia, Linnaeus; byssus large and calcified; British. Placuna, 
Bruguitre ; byssus atrophied in the adult. Hypotrema, dOrbigny. 
Carolia, Cantraine. Ephippiwm, Bolten. Placwnanomia, Broderip. 
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