Fam. 16. Cyrenidae.—Siphons short, foot large, no byssus; shell equivalve, subtriangular, with periostracum, hinge with two or three cardinals, laterals present; animal hermaphrodite, viviparous. Fresh or brackish water. Jurassic——. Genera: Cyrena, Corbicula (subg. Batissa, Velorita), Sphaerium (= Cyclas), Pisidium, Galatea, Fischeria.
The families Cyrenellidae (single genus, Cyrenella) and Rangiidae (single genus, Rangia) are probably to be placed here.
Sub-order II. Tellinacea.—Siphons long, separate, foot and labial palps very large, pallial sinus deep, two adductor muscles.
Fam. 1. Tellinidae.—External branchial fold directed dorsally, foot with byssogenous slit, but no byssus, branchiae small; shell compressed, equivalve, ligament external, at least two cardinals in each valve, laterals variable. Cretaceous——. Principal genera: Tellina (with many sections), Gastrana.
Fam. 2. Scrobiculariidae.—Animal as in Tellina; shell orbiculate or long oval, equivalve, hinge-teeth weak, ligament in an internal cavity. Tertiary——. Principal genera: Scrobicularia, Syndosmya, Theora, Cumingia, Semele.
Fam. 3. Donacidae.—External branchial fold directed ventrally; shell equivalve, subtriangular, solid, smooth, two or three cardinals in each valve, laterals variable, ligament external. Jurassic——. Genera: Donax, Iphigenia, Isodonta.
Fig. 306.—Tellina rastellum Hanl., East Indies.>
Fam. 4. Tancrediidae.—Shell donaciform, ligament external, cardinals usually two in each valve, posterior laterals strong. Trias——. Genera: Tancredia (Secondary strata), Hemidonax.
Fam. 5. Cardiliidae.—Shell heart-shaped, hinge as in Mactridae, posterior adductor resting on a myophore or shelf. Single genus, Cardilia. Tertiary——.