34 THE LIFE OF THE MOLLUSCA 
the anterior adductor superfluous, with the result 
that it tends proportionately to dwindle and dis- 
appear, leaving the enlarged posterior muscle to do 
the work of both, as in the Oyster and Scallop 
(Plate XVI., Fig. 3, A-D). Bivalves with two 
adductor muscles are termed “ Dimyaria”; and if the 
latter are of equal size, are said to be “Isomyarian”; 
or, if unequal, “ Heteromyarian”; whilst those 
having only one are called ‘‘ Monomyaria.” 
The Pelecypoda are subdivided, according to the 
structure of the gills, into four orders: 
Order I.: PROTOBRANCHIA (Plate XVIL., Figs. 1-3), 
in which the gill-filaments take the form of flattened, 
leaf-like expansions (Aspidobranch, Plate IV., Fig. 
5,4). To this order the Nut Shells (Nucula) and their. 
allies belong, and probably also the earliest known 
Bivalves, forms now quite extinct, that have been 
termed “‘ Palzoconcha.” With few exceptions they 
have a taxodont hinge. 
Order II.: FILIBRANCHIA (Plate XVIL., Figs. 5-17), 
having long, parallel gill-filaments, the ends of which 
are folded up, forming a W in section, and locked 
together by ciliated and sometimes interlamellar 
junctions (Plate IV., Fig. 5, B). To this order the 
Ark Shells, Mussels, and Pectens belong. 
Order III.: EULAMELLIBRANCHIA (Plates XVIII, 
XIX., and XX., Figs. 1-19), in which the elongated 
filaments are yet longer, more folded up, and per- 
manently united at intervals till they form a com- 
