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 XVII., Figs. 1-3), 

 in which the gill-filaments take the form of flattened, 

 leaf-like expansions (Aspidobranch, Plate IV., Fig. 

 5, A). 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 " Palseoconcha." With few exceptions they 

 have a taxodont hinge. 



Order II. : Filibranchia (Plate XVII., 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- 



