f>78 ZOOLOGY SECT. 



absent or formed by groups of interlocking cilia : interlamellar 

 junctions are either absent or non-vascular. As a rule there are 

 two adductor muscles, but the anterior may be greatly reduced or 

 absent. 



Including the Noah's ark shell (Area), Sea-mussel (Mytilus, 

 Fig. 595), Anomia, Trigonia, &c. 



ORDER 3. PSEUDO-LAMELLIBRANCHIA. 



Pelecypoda in which the gills are plaited so as to present 

 vertical folds : the interfilamentar junctions may be ciliary or 

 vascular : the interlamellar junctions vascular or non-vascular. 

 There is a single large (posterior) adductor muscle. The shell is 

 frequently inequivalve. 



Including the Scallop (Pecten, Fig. 585), Oyster (Ostrea), Pearl 

 Oyster (Meleagrina), Lima, Pinna, &c. 



ORDER 4. EULAMELLIBRANCHIA. 



Pelecypoda in which the gill-filaments are united by vascular 

 interfilamentar and interlamellar junctions, firm, basket-like gills 

 being the result : the gills may be smooth or plaited. There are 

 two equal-sized adductor muscles. 



Sub-order a. Integripalliata. 



Eulamellibranchia in which the siphons are small or absent 

 and the pallia! line on the shell is entire. 



Including the Fresh- water mussels (Anodonta and Unio). 



Sub-order b. Sinupalliata. 



Eulamellibranchia in which the siphons are of considerable 

 size, and the pallial line is inflected to form a sinus. 



Including the Cockle (Gardium), My a, Pholas, Teredo (Ship- 

 worm), Aspergillum, &c. (Figs. 587-590). 



ORDER 5. SEPTIBRANCHIA. 



Pelecypoda in which the gills are reduced to a horizontal 

 muscular partition. There are two adductor muscles. 

 Including Poromya, Cuspidaria, &c. 



Systematic Position of the Examples. 



Anodonta and Unio are two genera belonging to the family 

 Unionidce, sub-order Integripalliata, order Eulamellibranchia. 



Their complex basket-like gills are alone sufficient to place 

 them among the Eulamellibranchia. The incomplete ventral 

 siphon and the correlated entire pallial line (see p. 666) indicate 

 their position among the Integripalliata. The regular shell, with 



