456 EULAMELLIBRANCHIATA— MYACEA chap. 



The four succeeding families require special study in a work 

 on Palaeontology. 



Fam. 5. Monopleuridae. — Shell very inequi valve, left valve 

 operculiform, right conical or spiral, fixed at the apex, ligament 



prolonged in external grooves. Cretaceous . Genera ; 



Monopleura^ Valletia. 



Fam. 6. Caprinidae. — Shell very inequivalve, thick, free or 

 fixed by apex of right valve, which is spiral or conical, left valve 

 spiral or not, often perforated by radial canals from the umbo to 



the free margin. Neocomian and Cretaceous . Principal 



genera : Plagioptychus^ Caprina, Ichthyosarcolites^ Caprotina, 

 Polyconites. 



Fam. 7. HippuritidaeQ = Rudistae). — Shell very inequivalve, 

 externally as in Caprinidae, umbo central in left valve, no ligar 

 ment proper, left valve with strong hinge-teeth and grooves, two 

 adductor impressions on prominent myophores, shell structure 

 of the two valves differing : Cretaceous only. Single genus, 

 Hippurites (Fig. 309, B). 



Fam. 8. Radiolitidae. — Shell inversely conical, biconical, or 

 cylindrical, general aspect of Hippurites^ umbo of left valve 

 central or lateral, right valve with a thick outer layer, often 

 foliaceous, umbonal cavity partitioned off by laminae : Creta- 

 ceous only. Genera : Radiolites^ Biradiolites. 



Sub-order V. Myacea. — Branchiae much folded back, man- 

 tle edges usually with three openings, foot compressed, siphons 

 large, united or not, two adductor muscles, pallial line variable. 



Fam. 1. Psammohiidae. — Siphons long, not united, foot large, 

 not byssiferous; shell equivalve, long, oval, slightly gaping at 

 the ends, ligament external, prominent, two cardinal teeth in 



each valve, no laterals, a deep pallial sinus. Jurassic . 



Genera : Psammohia^ Solenotellina^ Sanguinolaria^ Asaphis, 

 Ulizia, Quenstedtia (Jurassic). 



Fam. 2. Myidae. — Pedal orifice small, siphons long, united 

 in great part; shell inequivalve, gaping at one or both ends, 

 periostracum more or less extensive, ligament internal, resting 



on a prominent shelf; hinge-teeth variable. Cretaceous . 



Genera : Mya^ Tugonia^ Sphenia^ Corhula^ Lutraria (for which 

 latter some propose a separate family). 



Fam. 3. Solenidae. — Foot long, powerful, more or less cylin- 

 drical, no byssus, siphons usually short, united or not, branchiae 



