EULAMELLIBRANCHIATA PHOLADACEA 



457 



narrow ; shell equivalve, long and narrow, gaping at both ends, 

 with periostracmn, umbones flattened, ligament external, hinge 



teeth variable. ? Devonian . Genera : Solecm'tus, Pharella, 



Pharus, C id tell us, Siliqua, Ensis, Solen, 

 Orthonota (?), Palaeosolen (?). 



Fam. 4. Glycimeridae. — Pedal orifice 

 very narrow, siphons long, united in great 

 part, often covered with periostracmn ; 

 shell more or less equivalve, gaping at 

 both ends, hinge toothless or with two 

 weak cardinals, ligament external ; animal 



free or perforating. Cretaceous- . 



Genera : Glycimeris, Saxicava, Cyrtodaria. 



Fam. 5. Gastrochaenidae. — Foot small, 

 cylindrical, no byssus, branchiae narrow, 

 siphons long ; shell perforating or 

 cemented to a shelly tube, gaping widely 

 on the anterior and ventral sides, no 

 hinge teeth, a deep pallial sinus. Cre- 

 taceous- . General : Gastrochaena, 



Fistulana (tube with a median diaphragm, 

 perforated by the siphons). 



Sub-order VI. Pholadacea, — Mantle 

 edges largely closed, siphons long, united, 

 foot short, truncated, disc-shaped, liga- 

 ment absent, two adductor muscles ; 

 animal perforating. 

 ■ Fam. 1. Pholadidae. — Organs con- 

 tained within the valves, ctenidia 

 prolonged into the branchial siphon, 

 shell more or less gaping, thin, dorsal 

 margin in part reflected over the um- 

 bones, one or more dorsal accessory pieces, fig. bio.— Teredo navaiis L.-. 

 no hinge teeth, an interior apophysis V, valves of shell ; T, tube ; 



P ^ , ^ . P, pallets ; SS, siphons. 



proceeding from the umbonal cavity. (After Mobius.) 



Jurassic- . Genera: Phol((s, Tidona, 



Pholadidea (posterior extremity of the valves prolonged by a 

 corneous appendage, a passage to the long tube of Teredo), 

 Jouannetia, Al/lojjhaga, Martesia ; Teredina (Eocene). 



Fam. 2. Teredinidac — Auimnl vermiform, ctenidia mainly 



