XVI SEPTIBRANCHIATA 459 



exterior often granulose, ligament often with ossicle, hinge tooth- 

 less or with lamellae. Jurassic . Genera: Anatina^ Plecto- 



mya (Secondary), Periploma, CocJdodesma^ Thracia, Tyleria, 

 Alicia, Asthenothaei'us. 



Fam. 8. G-rammi/sndae. — Shell equivalve, oval, ligament 

 external, cardinal margin straight, toothless, pallial line com- 

 plete. Palaeozoic . Principal genus, Grrammysia, with 



many other genera of toothless hinge, but whose exact afQnities 

 are uncertain. 



Fam. 9. Praecardiidae. — Shell thin, equivalve or not, 

 radiately ribbed, margins dentated, subumbonal area as in Aixa, 



hinge toothless. Palaeozoic . Principal genus, Praecar- 



dium. 



Fam. 10. Plioladomyidae. — A fourth pallial orifice, siphons 

 very long, united, foot small ; shell thin, equivalve, with radiat- 

 ing ribs, ligament external, hinge toothless, pallial line sinuate. 

 Jurassic . Single genus, Pholadomya. 



Fam. 11. Clavagellidae. — Foot rudimentary, siphons long, 

 united, contained in a long calcareous tube ; shell small, one or 

 both valves soldered in the tube, tube with a centrally fissured 

 disc at the anterior end, more or less frilled at the posterior 



end. Cretaceous . Genera: Clavagella, Brechites (^ = Asper- 



gillurri). 



Order V. Septibranchiata 



Mantle edges united at three points, branchiae replaced by 

 a muscular septum extending from the anterior adductor to the 

 point of separation of the siphons, septum with symmetrical 

 orifices. 



Fam. 1. Poromyidae. — Branchial septum with groups of 

 lamellae between the orifices, labial palps large, foot long and 

 narrow, siphons short, papillose, separated, animal hermaph- 

 rodite; shell small, slightly inequivalve, rounded or oblong, 

 nacreous within. Eocene . GQnQv?i'. Poromya, Silenia. 



Fam. 2. Cuspidariidae. — Siphons long, united in part, 

 foot short, animal dioecious; shell small, slightly inequivalve, 

 rostrate, not nacreous, each valve with ligamentary cartilage 

 spoon-shaped, with a calcareous ossicle, cardinal and lateral 



teeth present or absent. Jurassic . Single genus, Cuspi- 



daria, with many sections. 



