324 



MOLLUSCA 



Order II. Heteroconchiae. 



Gills always lamellar, their outer surface frequently plaited. Hinge- 

 in rare cases (Anodonta) lost by degeneration is heterodont or modified 

 from a heterodont condition. The mantle edges but rarely free in their 

 whole extent; siphons usually present, but in some so 

 small (Integripalliata) as to cause no sinus in the pallial 

 line; in others (Sinupalliata) large, the pallial line hav- 

 ing a marked sinus. Anterior and posterior adductors 

 equally developed. 



Sub Order I. INTEGRIPALLIATA. UNIONID*:, fresh- 

 water mussels; hundreds of species in the Mississippi basin 

 afford material for pearl buttons. In some pearls of value 

 are occasionally found. Unio* Anodonta* The tropical 

 TRIDACNHXE includes the largest acephalan, Tridacna gigas, 

 the shell of which may be four feet long and weigh three 

 hundred pounds. The heart shells (CARDIID/E Cardium,* 

 Serripes*) and ASTARTID^, marine, and the fresh-water 

 CYCLADID^E (Cyclas, Pisidi'um*), about the size of peas, 

 belong here, as probably do the extinct RUDISTID^: of the 

 cretaceous, in which the attached right valve is drawn out 

 into a long cone, the left fitting like a lid over the small cavity. 



Sub Order II. SINUPALLIATA. The VENERID^: with 

 swollen shells, Vcmts mcrcenaria* quahog, MACTRID.E, hen 



C 



FIG. 327. 



12 



FIG. 328. 



FIG. 327. A, Saxicava arctica; B, Astarte sulcata; C, Siliqua costata (from Binney- 

 Gould). 



FIG. 328. Teredo navalis, ship-worm (after Grobben). The mantle opened back 

 to the siphons, i, foot; 2, shell; 3, liver; 4, 5, ventricle and auricle of heart; 6, 

 gonads; 7, gills; 8, cloaca; 9, gill chamber; 10, n, anal and branchial siphons; 12, 

 pallets on siphons. 



clams, and the TELLINID^; (Tellina* Macoma*), have short siphons. In 

 others the siphons are so large that they cannot be entirely retracted within the 

 shell, as in the MYID^E, represented in all northern seas by the long clam, Myc 

 arcnaria,* and in the razor clams (SOLENID^E; Solcn, Ensatella*). The allied 



