//. ACEPIIALA: IIETEliOCOXCHI^i:. 



501 



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

 taxodont Nuculid.e have cteiiidia, a broad foot, pleural and cerebral gan- 

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

 show them extremely primitive. NucuJa* Leda* Yoldia* The Arcid^e 

 are also taxadont, but iilibranch. Hca'pliarca* Argina* SoLEMYiDiE. 



Sub Order II. AXISOMYARIA. 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. Modiohi* Pinna * Mytilus eduUs, abundant on 

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

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

 Caspian through central Europe. Lithodomus '" bores into stone. Tlie 

 AvicuIjID^ of warm seas have wing-like projections either side of tlie umbo. 

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

 The OsTR^iD^ and the PectiniDjE are monomyarian. The Ostraeidee, or 

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

 Ostixea virginiaiia differs from the European species in having the sexes 

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

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



Order II. HeteroconchicE. 



Gills always lamellar, their outer surface frequently plaited. 

 Hinge — in rare eases {Anodonta) lost by degeneration — is lietero- 

 dont or modified from a lieterodont conditiou. The mantle edges 

 hut rarely free in their whole extent; siplioiis usually present, but 

 in some so small (Integrii^alliata) a.s 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 (Xaiad;e) 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 jiearl buttons. In some pearls of value are occasionally found. Unio, 

 Anodonta. The tropical Tridacnid.i?, with small siphons, includes the 



Fig. 35B.^^1, Waj-icuiu arctaii, B, A^tititi siihala; V. SiUqua ciititata. (Fmm Binni-y- 



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

 long and weigh three hundred pounds. The heart shells (C'Aunnn.E— 



