448 FILIBRANCHIATA chap. 



branchial filaments pointing dorsall}^, the other ventrallj; mantle 

 edges in great part united postero-ventrally, a single siphonal 

 orifice with two very long tentacles, foot proboscidiform, with a 

 round denticulate disc at the end; shell equi valve, resembling a 

 SoUn^ with a strong corneous periostracum ; no hinge-teeth, liga- 

 ment internal. Single genus, SoUnomya. (?) Cretaceous . 



Order II. Filibranchiata 



Rows of branchial filaments parallel, pointing ventrally, 

 reflected, and provided with interfilamentary ciliated junctions, 

 foot usually with a well-developed byssogenous apparatus. 



Sub-order I. Anomiacea. — Heart dorsal to the rectum, a 

 single aorta, foot small, anterior adductor very small ; shell 

 ostreiform, no hinge-teeth, fixed by a calcified byssus traversing 

 the right valve (Fig. 173, p. 262). 



Fam. Anomiidae. — Jurassic . Genera : Ayiomia^ Pla- 



cunanomia ; Carolia (Eocene), Plaeiina ; Hypotrema (Jurassic), 

 Placiinopsis (Oolite). 



Sub-order II. Arcacea. — Mantle edge open, both adductors 

 well developed, heart with two aortae, branchiae free, without 

 interlamellar junctions, no siphons ; renal and generative aper- 

 tures distinct. 



Fam. 1. Arcadae. — Mantle edge with composite eyes; shell 

 round or trapezoidal, solid, often with stout bushy periostracum ; 

 ligament often external, on a special area ; hinge with numer- 

 ous lamelliform teeth. Ordovician . 



Principal genera; Area (incl. Barhatia^ 

 SeapJmla^ and Cucullaea)^ heart dorsal to 

 rectum ; Pectunculus^ Glomus, Limopsis ; 

 Trinaeria and Nuculina (Tertiary). 



Fam. 2. Trigoniidae. — Foot large, 

 hatchet -shaped, with ventral disc ; no 

 byssus, mantle edge with ocelli ; shell 

 sub-trigonal, hinge-teeth few, strong; in- 



FiG. 301. — THgonia pecti- terior violet - nacreous. Devonian . 



nTw^'"''''' ^^^''^^' Grenera: Trigonia ; Myoplioria and Schiz- 

 odus (Trias), Oyrtonotus (Devonian). 

 Sub-order III. Mytilacea. — Mantle edges fused at one point, 

 anal orifice distinct, anterior terminal adductor small, one aorta, 



