XVI EULAMELLIBRANCHIATA — TELLINACEA 453 



branchial orifice on the buccal margin, foot long, broadened, 



with a byssns, animal usually viviparous. Tertiary . 



Genera : JErycina^ Kellia^ Pythina^ Lasaea^ Lepton. 



Fam. 15. Gialeommidae. — Mantle edges more or less re- 

 flected over the shell, apertures and foot as in Erycinidae ; shell 



thin, equilateral, hinge with few teeth or none. Tertiarj' . 



Genera : G-aleomma^ Scintilla^ Sportella^ Chlamydoconcha^ Hinds- 

 iella, Ephippodonta (Fig. 32, p. 81). 



Fam. 16. Cyrenidae. — Siphons short, foot large, no bys- 

 sus ; shell equivalve, subtriangular, with periostracum, hinge 

 with two or three cardinals, laterals present ; animal hermaphro- 

 dite, viviparous. Fresh or brackish water. Jurassic . Gen- 

 era: Cyrena^ Corhicula (subg. Batissa^ Velorita)^ SpTiaerium 

 (= Oyclas)^ Pisidium, Galatea^ FiscJieria. 



The families Oyrenellidae (single genus, Cyrenelld) and 

 Rayigiidae (single genus, Rangid) are probably to be placed here. 



Sub-order II. Tellinacea. — Siphons long, separate, foot and 

 labial palps very large, pallial sinus deep, two adductor muscles. 



Fam. 1. Tellinidae. — External branchial fold directed 

 dorsally, foot with byssogenous slit, but no byssus, branchiae 

 small ; shell compressed, equivalve, ligament external, at least 



two cardinals in each valve, laterals variable. Cretaceous . 



Principal genera: Tellina (with many sections), Gastrana. 



Fajsi. 2. Serohiculariidae. — Animal as in Tellina; shell orbic- 

 ulate or long oval, equi- 

 valve, hinge-teeth weak, 

 ligament in an internal 

 cavity. Tertiary . 



Principal genera : Scrohi- 

 cularia^ Syndosmya^ The- 

 ora^ Camingia^ Semele. 



Fam. 3. Donacidae. — 

 External branchial fold ^^^ 3^^ _ ^^^^.^^ ^^^^^^,^^^ ^^^^ ^ ^^^^ ^^^.^^ 

 directed ventrally ; shell 



equivalve, subtriangular, solid, smooth, two or three cardinals 

 in each valve, laterals variable, ligament external. Jurassic 

 . Genera: Donax^ Iphigenia^ Isodonta. 



Fam. 4. Tancrediidae. — Shell donacif orm, ligament external, 

 cardinals usually two in each valve, posterior laterals strong. 

 Trias , Genera: Tancredia (Secondary strata), Hemidonax. 



