xii PHYLUM MOLLUSCA 647 



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

 frequently inequivalve. 



Including the Scallop (Pecten), Oyster (Ostrea), Pearl oyster 

 (Meleagrina), Lima, Pinna, &c. 



ORDER 4. EULAMELLIBRANCHIA. 



Pelecypoda in which the gill-filaments are united by vascular 

 inter-filamentar and inter-lamellar 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. Tntegripalliata. 



Eulamellibranchia in which the siphons are small or absent, 

 and the pallial 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 (Cardiuni), My a, Pholas, Teredo (Ship- 

 worm), Aspergillum, &c. 



ORDER 5. SEPTIBRANCHIA. 



Pelecypoda in which the gills are reduced to a horizontal 

 muscular partition. There are two adductor muscles. 

 Including Poromya, Cuspidaria, &c. 



P Systematic Position of the Examples. 



Anodonta and Unio are two genera belonging to the family 

 Unionidae, sub-order Asiplwniata, 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. 650) indicate 

 their position among the Asiplwniata. The regular shell, with 

 thick brown periostracum and large external ligament, the elon- 

 gated branchial or inhalant aperture, the long, compressed foot, 

 nd the absence of a byssus, place them among the Unionidae. 

 nodonta is distinguished from Unio by the absence of hinge- 

 eth. 



3. GENERAL ORGANISATION. 



The most important variations in structure in the present class 

 connected with modifications of the gills, the foot, the muscular 



stem, and the siphons. With the structure of the muscles and 

 the siphons are correlated important variations in the shell 



T T f 



- 



