Mantle not extensively closed; two pallial sutures and two
well-developed siphons. Gills smooth. Foot compressed and elongated.
Labial palps very large. Dimyarian; pallial line with a deep sinus.
Fam. 1. _Tellinidae._--External gill-plate directed upwards; siphons
separate and elongated; foot with byssus; palps very large; ligament
external. _Tellina_; British. _Gastrana_; British. _Capsa. Macoma._
Fam. 2. _Scrobiculariidae._--External gill-plates directed upwards;
siphons separate and excessively long; foot without byssus.
_Scrobicularia_; estuarine; British. _Syndosmya_; British.
_Cumingia_.
Fam. 3. _Donacidae._--External gill-plate directed ventrally;
siphons separate, of moderate length, anal siphon the longer.
_Donax_; British. _Iphigeneia._
Fam. 4. _Mesodesmatidae._--External gill-plate directed ventrally;
siphons separate and equal. _Mesodesma. Ervilia_; British.
[Illustration: FIG. 29.--The same animal as fig. 28, with its foot
and siphons expanded. Letters as in fig. 28. (From Gegenbaur.)]
Fam. 5. _Cardiliidae._--Shell very high and short; dimyarian;
posterior adductor impression on a prominent apophysis. _Cardilia._
Fam. 6. _Mactridae._--External gill-plate directed ventrally;
siphons united, invested by a chitinous sheath; foot long, bent at
an angle, without byssus. _Mactra_; British (figs. 28, 29).
_Mulinia. Harvella. Raeta. Eastonia. Heterocardia. Vanganella._