SUBMYTILACEA. 



723 



geria, of CEhlert, in which the shell (fig. 597, d) is long-oval and 

 very inequilateral, with small subterminal beaks, and a concentri- 

 cally-striated surface. The ligament is external ; both valves have 

 an elongated posterior lateral tooth ; and the right valve has a 

 cardinal tooth which fits into a corresponding socket in the left 

 valve. 



Family 5. Carditid^e. — In this family the mantle-lobes are free, 

 the foot is byssiferous or is grooved interiorly, and there are two 

 adductor muscles. The shell is equivalve, solid, cordate, oval, or 

 transversely elongated, and generally adorned with radiating ribs. 

 The pallial line is entire ; the ligament is almost always external ; 

 and the hinge is massive, and supports one or two oblique cardinal 

 teeth and sometimes lateral teeth as well. The members of this 

 family are marine in habit, and the two most important recent 

 genera are Ve?iericardia and Cardita, which are closely related to 

 one another, and are separated principally by characters connected 

 with the living animal, the former having a large foot which is 

 grooved and non-byssiferous, while in the latter the foot is short 

 and secretes a byssus. 



In both Venericardia and Cardita the shell (fig. 598) is massive, 

 more or less cockle-shaped, inequilateral, and adorned with radiat- 



Fig. 598. — Venericardia {Cardita) planicosta. Eocene Tertiary. 



ing ribs, the ventral margin being denticulated or crenulated. The 

 ligament is external, and the hinge-plate is thick and furnished with 

 powerful cardinal and variably developed lateral teeth. The genus 

 Cardita ranges in time from the Trias to the present day; while the 

 species of Venericardia abound in the Cretaceous and Tertiary rocks, 

 and a few forms still survive. 



With the preceding may be associated, provisionally at any rate, 

 a number of wholly extinct genera. Of these the genus Pleuro- 

 phorus (fig. 599) possesses an oblong shell, with anterior, almost 

 terminal beaks, and a massive hinge. Each valve has two diverg- 

 ing cardinal teeth, with a single elongated lateral tooth placed 

 posteriorly ; and the anterior adductor impression is very deep, 



