//. AGEPHALA: HETEROCONCHI^E. 367 



Sub Order I. DIMYARIA. Two equally developed adductors. The 

 taxodont NUCULID^E have ctenidia, a broad foot, pleural and cerebral gan- 

 glia separate, and gonads emptying through the nephridia, all points which 

 show them extremely primitive. Niicula* Leda* Yoldia.* The ARCID^E. 

 are also taxadont, but filibranch. Scapharca* Argina* SOLEMYID^E. 



Sub Order II. ANISOMYARIA. Anterior adductor rudimentary 

 (Heteromyaria) or wanting (Monomyaria). With the exception of the 

 isodont SPONDYLID^E, all the families lack a hinge (dysodont). To the 

 Heteromyaria belong the MYLILIDJE, or mussels, with strong byssus and 

 shells pointed anteriorly. Modiola* Pinna* Mytilus edulis, abundant on 

 our mud flats ; eaten in Europe, but occasionally poisonous. Dreissenia 

 polymorpha, a brackish and fresh-water species, has spread from the 

 Caspian through central Europe. Lithodomus * bores into stone. The 

 AVICULID^E of warm seas have wing-like projections either side of the umbo. 

 The pearl oysters of the East and West Indies (Meleagrina) belong here. 

 The OSTR^EID^E and the PECTINID.E are monomyarian. The Ostrasidas, or 

 oysters, usually become attached by the right valve. Our American 

 Ostrcea mrginiana differs from the European species in having the sexes 

 separate. The Pectin idas, or scallops, are free-swimming and are well 

 known for their highly developed green eyes on the edge of the mantle. 



Order II. Heteroconchiae. 



Gills always lamellar, their outer surface frequently plaited. 

 Hinge in rare cases (Anodonta) lost by degeneration is hetero- 

 dont 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 having a marked 

 sinus. Anterior and posterior adductors equally developed. 



Sub Order I. INTEGRIPALLIATA. The UNIONID.E (Naiadee) include 

 the fresh- water mussels, of which hundreds of species occur in the Missis- 

 sippi basin, some of which are markedly iridescent and afford material 

 for pearl buttons. In some pearls of value are occasionally found. Unio,* 

 Anodonta* The tropical TRIDACNID.E, with small siphons, includes the 



FIG. 358. .A, Saxicava arctica; B, Astarte sulcata ; C. Siliqua costata. (From Binney- 



Gould.) 



largest Acephala, Tridacna gigas, the shell of which may be four feet 

 long and weigh three hundred pounds. The heart shells 



