SEPTIBRANCHIATA 459 



often granulose, ligament often with ossicle, hinge toothless or 



with lamellae. Jurassic . Genera : Anatina, Plectomya 



(Secondary), Ferijjloma, Cochlodesma, Thracia, Tyleria, Alicia, 

 Asthenothaerus. 



Fam. 8. Grammysiidae. — Shell equivalve, oval, ligament 

 external, cardinal margin straight, toothless, pallial line complete. 



Palaeozoic . Principal genus, Grammysia, with many other 



genera of toothless hinge, but whose exact affinities are uncertain. 



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

 ribbed, margins dentated, subumbonal area as in Area, hinge 

 toothless. Palaeozoic . Principal genus, Praecardium. 



Fam. 10. Pholadomyidae- — A fourth pallial orifice, siphons 

 very long, united, foot small ; shell tliin, equivalve, w^ith radiating 

 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- 



gillum). 



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 hermaphrodite ; 

 shell small, slightly inequivalve, rounded or oblong, nacreous 

 within. Eocene . Genera : 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, Cuspidaria, with many sections. 



