712 LAMELLIBRANCHIATA. 



sembles Myalina, but it is destitute of the thick striated hinge- 

 plate of the latter, and the ligament is external. 



Order IV. Arcacea. 



In this order the mantle-lobes are completely separate, or may 

 form two respiratory siphons : the foot is byssiferous or grooved ; 

 and there are two adductors. The shell is ordinarily equivalve, and 

 the hinge is furnished in both valves with a series of equal and 

 similar teeth. This division of Bivalves includes the two families 

 of the Areidee and Nuculidce, both of which have representatives in 

 rocks as ancient as the Ordovician, at any rate. 



Family i. Arcidce. — In this family the muscular impressions 

 are nearly equal, the foot is large, bent, and deeply grooved, and 

 the mantle-lobes are separate. The shell is equivalve, and the 

 hinge is long and carries numerous comb-like teeth. The ligament 

 is usually external, attached to a striated area below the beak, but 



it may be internal, and contained within 

 a single groove. The family of the Ar- 

 cidce. appears to be represented in the 

 Upper Cambrian deposits (GlyJ>tarca) 

 and is largely developed in existing seas. 

 In the genus Area (fig. 582) the shell 

 is transversely elongated and inequilateral, 

 the surface being generally ornamented 

 with radial ribs or striae. The hinge-line 

 is straight, and is furnished with numerous 

 transverse equal teeth. The beaks are 

 remote from one another, and are separ- 

 „„„,. ated by an oval or lozenge-shaped, striated 

 ligamental area. Taken as a whole, the 

 genus Area ranges from the Upper Cambrian to the present day, 

 the oldest types (Glyfitarca) appearing in the Lower Tremadoc 

 beds. 



The genus Area has been broken up into numerous sub-genera, of 

 which the two most important are Carbonarca and Isoarca. The former 

 of these comprises some types from the Carboniferous Limestone, in 

 which the beaks are tumid and incurved, and the hinge has two strong 

 oblique teeth in front, and numerous transverse teeth behind. The forms 

 included under the head of Isoarca belong to the Jurassic and Cretaceous 

 rocks, and have an elongated, very inequilateral shell, with the beaks 

 near the anterior end. The beaks are inflated and incurved, and the 

 hinge-line is slightly curved, and has a row of transverse teeth on each 

 side. 



Closely allied to Area is the genus Cueullcea, in which the shell is 

 trapezoidal and ventricose ; the hinge-line is straight ; and the hinge- 



