452 EULAMELLIBRANCHIATA — SUBMYTILACEA chap. 



labial palps large ; shell irregular, free or fixed, no hinge-teeth. 

 Fluviatile, recent only. Genera : Aetlieria^ Mlllleria., Bartlettia. 



Fam. 7. Unionidae. — Foot large and thick, no byssus, anal 

 siphon short, branchial orifice complete or not, siphon present or 

 absent, embrj'O of certain groups passing through a glocliidium 

 stage (p. 146) ; shell equivalve, sometimes very thick, nacreous 



within, hinge variable. Fluviatile. Jurassic . Principal 



genera : Unio (subg. Arconaia)^ Monocondylaea^ Pseudodon, Ano- 

 donta, Solenaia^Mycetopus^ Mutela^ Spatha, Hyria^ Castalia, Leila. 



Fam. 8. Dreissensiidae. — Both siphons prominent, foot tongue- 

 shaped, byssiferous ; shell mytiliform, with small internal septum. 

 Genera: Dreissensia; Dreissensiomya (Tertiary). The common 

 Dreissensia polymorplia Pall, was distributed over large parts of 

 Europe in later Tertiary times. From unknown causes it died 

 out, and has during the past two hundred years been regaining 

 its position, migrating N. and W. from its original habitat, the 

 Caspian, by the Volga and its Oka confluent. 



Fam. 9. Modiolopsidae. — Shell mytiliform, ligament exterior, 



hinge-teeth small, rather numerous. Palaeozoic . Principal 



genera : Modiolopsis, Oyrtodonta^ Mytilops^ Ptychodesma. 



Fam. 10. Lucinidae. — Anal orifice sometimes with a siphon, 

 branchial orifice complete or not, sometimes a single branchia ; 

 foot very long, vermiform, no byssus, anterior adductor long ; 

 shell rounded, equivalve, blanched, hinge with two cardinals 

 and two laterals in' each valve, sometimes toothless, ligament 



more or less internal. Silurian . Principal genera : Lucina^ 



Corbis^ Axinus, Diplodo7ita, Montacuta. 



Fam. 11. Ungidinidae. — Anal orifice complete, foot vermi- 

 form, no byssus, two branchiae ; shell equivalve, subcircular, hinge- 

 teeth variable, no laterals, adductor impressions long, continuing 

 the pallial line. Tertiary . Single genus, Ungulina. 



Fam. 12. Unicardiidae. — Shell equivalve, round or oval, 

 cardinal shelf large, a single cardinal in each valve, ligament 

 external. Carboniferous — Cretaceous. Genera : Ujiicardium, 

 Scaldia^ Pseudedmondia. 



Fam. 13. Kelly ellidae. — Anal siphon prolonged, no marked 

 branchial orifice ; shell very small, oval or round, anterior lateral 



very strong, under the cardinal. Eocene . Genera : Kelly- 



ella ; Allopagus and Lutetia (Tertiary), Turtonia. 



Fam. 14. Erycinidae. — Mantle edges with three apertures, 



