Ligament wanting; shell gaping, with a styloid apophysis in the
umbonal cavities. Gills prolonged into the branchial siphon. Mantle
largely closed, siphons long, united. Foot short, truncated, discoid,
without byssus.
Fam. 1. _Pholadidae._--Shell containing all the organs; heart
traversed by the rectum; two aortae. Shell with a pallial sinus;
dorsal region protected by accessory plates. _Pholas_; British.
_Pholadidea_; British. _Jouannetia. Xylophaga_; British. _Martesia._
Fam. 2. _Teredinidae._--Shell globular, covering only a small
portion of the vermiform body; heart on ventral side of rectum; a
single aorta; siphons long, united and furnished with two posterior
calcareous "pallets." _Teredo_; British. _Xylotrya._