268 THE LAMELL1BRANCHIA 



stage (Fig. 242). Genera Unio, Retzius ; shell thick, the hinge toothed. 

 This genus includes more than a thousand species, the majority from the 

 northern hemisphere. A nodonta, Lamarck ; shell thin; the hinge without 

 teeth ; British. Pseudodon, Gould. Quadrula, Rafi nesque. Arconaia, Conrad. 

 Motwcondylaea, d'Orbigny. Solenaia, Conrad. Mycetopus, d'Orbigny ; 

 foot cylindrical, with a terminal swelling ; South America. FAMILY 25. 

 MUTELIDAE, Gray. This family differs from the Unionidae in having two 

 pallial sutures and a distinct branchial orifice ; the shell is never fur- 

 nished with lateral hinge teeth. Freshwater. Genera Mutela, Scopoli. 

 Pliodon, Conrad. Spatha, Lea. Iridina, Lamarck. Hyria, Lamarck. 

 Castalia, Lamarck. Aplodon, Spix. Plagiodon, Spix. FAMILY 26. 

 AETHERIIDAE, Adams. Shell irregular, generally fixed in the adult state. 

 Mantle with a single suture ; foot absent ; anterior adductor muscle 



FlQ. 241. 



Modiolaria trapezina, Lamarck, viewed from the left side ; the left mantle lobe is removed. 

 ad', ad", anterior and posterior adductor muscles ; br 1 , br", internal and external gill-plates ; 

 gl.p, foot gland ; o.a, o.b, anal and branchial orifices of the mantle ; o.by, byssal orifice of the 

 foot ; o.p, pedal orifice of the mantle ; p, foot ; jxil, labial palp ; re.p, posterior retractor muscle 

 of the foot. 



sometimes reduced or absent ; from fresh water. Genera Aetheria, 

 Lamarck ; anterior adductor well developed ; African. Mulleria, F^russac ; 

 no anterior adductor ; American. Bartlettia, Adams. 



SUB-ORDER 3. TELLINACEA. 



Eulamellibranchia in which the mantle is not extensively closed, 

 with two pallial sutures and two well-developed siphons ; the gills 

 smooth. The foot is compressed and elongated. The labial palps very 

 large. Diinyarian ; the pallial line has a deep sinus. 



FAMILY 1. TELLINIDAE, Deshayes. The external branchial plate 

 directed upwards (Fig. 206, H). The siphons separate and elongated. 

 Foot with a byssogenous apparatus. Palps very large. Ligament of shell 

 external. Genera Tellina, Linnaeus ; slightly inequivalve ; foot large ; 

 British (Fig. 190). Gastrana, Schumacher ; equivalve ; the foot slightly 

 developed ; British. Capsa, Bruguiere. Macoma, Leach. FAMILY 2. 

 SCROBICULARIIDAE, Adams. External gill -plate directed upwards. 

 Siphons separate and excessively long. Foot without a byssus. The 

 ligament partly internal, lodged in a concavity in the hinge. Genera 



