MONOMYARIA. 129 



Genus 19— PLAGIOSTOMA. —Zawiarc^i. 



Generic Character. — Shell subequivalve, free, somewhat 

 eared; base of the hinge transverse and straight; beaks some- 

 what remote; hinge without teeth; depression, for the re- 

 ception of the ligament, conical and partly internal. Species 

 all fossil, 



. Plagiostoma spinosum. — The Thorny Plagiostoma. 

 Plate XV. fig. 6. Subarcuated, the umbo of one shell 

 higher than that of the other, with longitudinal ribs, and 

 remote concentric rings. FossiL Britain. 



Genus 20 —LIMA. — Bruguiere. 



Generic Character. — Shell longitudinal, nearly equi valve, 

 with small ears ; valves gaping at the sides ; beaks separate ; 

 cavity for the ligament partly external; hinge without teeth. 



Lima vitrina. — The Glassy Lima. Plate XV. fig. 7. 

 Oblong-ovate; extremely pellucid, and pure snowy-white, 

 with longitudinal distinct striae; lower margin denticulated. 

 One half inch long. Inhabits the British seas. 



Genus 21— PEDUM Lamarck. 



Generic Character Shell inequi valve, subauriculate ; 



the lower valve tumid ; beaks unequal, separate ; hinge 

 without teeth; ligament partly external, inserted in an 

 elongated groove, in the internal surface of the beaks; 

 lower valve, with a notch near the posterior margin. 



Pedum spondyloideum. — The Spondylus-shaped Pe- 

 dum. Plate XV. fig. 9. Ovate, wedge-shaped, flat; the 

 upper valve with longitudinal striae, white, granulated, and 

 rough, slightly tinged with purple near the beak. Two 

 inches and a half long. Inhabits the Indian ocean, and the 

 seas of the Isle of France 



