74-8 LAMELLIBRANCHIATA. 



are Scrobicularia and Seme/e, both of which are represented by Ter- 

 tiary and Recent species. 



Order XIV. Anatinacea. 



This order includes marine Bivalves with a moderate - sized, 

 grooved, or byssiferous foot, distinct siphonal orifices, a single 

 branchia on each side, and two adductor muscles. The shell is 

 thin, usually with a pearly internal layer and a finely granulated 

 external layer. The hinge may or may not carry teeth, and the 

 pallial line is mostly sinuated, but may be entire. The principal 

 families included in this order are the Sok??iyidce, Arcomyidce, Anat- 

 inidcs, Gra?nmysiidce, Prceca?'diidce, Pholadomyidce, and Clavagellidtz. 



Family i. Solemyid^e. — This family includes the single genus 

 Soiemya, which ranges from the Devonian to the present day. In 

 this genus the shell is equivalve, inequilateral, transversely elongated, 

 obtuse and gaping at both ends, with the epidermis prolonged 

 beyond the ventral margin. The beaks are inconspicuous ; the 

 ligament is partly internal and partly external ; the hinge is tooth- 

 less, or has a single cardinal tooth in each valve ; and the pallial 

 line is obscure. The Devonian and Carboniferous genus Clino- 

 pistha appears to be related to Solemya. 



Family 2. Arcomyidce. — The forms included in this family are 

 all extinct, and have usually been included in the Pholado??iyidce, 

 from which they are distinguished by the finely granulated character 

 of the exterior of the shell. The shell is equivalve, inequilateral, 



Fig. 634. — Arco7iiya (Homomya) calceiformis, viewed from the dorsal side, two-thirds of the 

 natural size. Lower Jurassic rocks. (After Zittel.) 



very thin, with an edentulous hinge, but with a thickened cardinal 

 margin. The ligament is external and prominent, and the pallial 

 line is sinuated. The principal genera included in this family are 

 Arcomya (fig. 634), Goniomya, and Pleiiromya, all of which are con- 

 fined to the Secondary rocks, the first and last ranging from the 

 Trias to the Chalk, while Goniomya is Jurassic and Cretaceous. 

 Family 3. Anatinid^e. — In this family the mantle-lobes are more 



