SIPHONIDA. 503 



an external ligament, and has the beaks placed anteriorly. 

 A second is the Permian Pleurophorus (fig. 368, D), in which 

 the shell is oblong, with anterior beaks, two cardinal teeth 

 in each valve, and a single elongated lateral tooth placed 

 posteriorly, the anterior adductor impression being very 

 deep. The third is the characteristically Silurian genus 

 Cleidophorus, which has usually been regarded as identical 

 with the preceding, though apparently upon insufficient 

 grounds. The most peculiar feature in this genus is a 

 vertical internal ridge, which commences in front of the 

 beaks, and is continued downwards behind the anterior 

 adductor, and which leaves a deep slit in the cast of the 

 shell (fig. 368, E). 



Subdivision II. Sinupallialia. Eespiratory siphons large ; 

 pallial line indented. 



FAM. 14. VENERID^E. Shell regular, sub-orbicular or ob- 

 long ; ligament external ; hinge with usually three diverging 

 teeth in each valve. Animal usually free and locomotive ; 



Fig. 370. Left valve of Cytherea chione (after Woodward). A, Anterior margin; B, Pos- 

 terior margin ; c, Ventral margin or base, u, Umbo ; h, Ligament ; I, Lunule ; c, Cardinal 

 tooth ; t, t, Lateral teeth ; a, Anterior adductor ; a', Posterior adductor ; p, Pallial line ; *, 

 Pallial sinus, caused by the retractor muscles of the siphons. 



mantle with a rather large anterior opening ; siphons un- 

 equal, more or less united. Foot tongue-shaped, compressed, 

 sometimes grooved and byssiferous. The Veneridce are the 

 most highly organised of the Bivalves, and comprise some 



