448 FILIBRANCHIATA 



branchial filaments pointing dorsally, the other ventrally; 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 equivalve, resembling a 

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

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



Order II. Filibranchiata 



Kows of branchial filaments parallel, pointing ventrally, re- 

 flected, and provided with interfilamentary ciliated junctions, foot 

 usually with a w^ell-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: Anomia, Placuna- 



nomia ; Carolia (Eocene), Placuna ; Hy^otrema (Jurassic), Placwn- 

 opsis (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 (inch Barhatia, 

 Scaph%da, and Gucullaeci), heart dorsal to 

 rectum ; Pectuncidus, Glomus^ Lvmo])sis ; 

 Trinacria and Nucidina (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- 



^ o^i m ■ ■ ,■ terior violet - nacreous. Devonian . 



Fig. 301. — Tngonia joecti- 



nata Lam., Sydney, Genera : Trigonia ; Mi/ojjhoria and Schiz- 

 ' ' ■ odus (Trias), Cyrtonotus (Devonian). 



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

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



