MONOMYARIA. 129 



Genus 19.— PLAGIOSTOMA.— ZawjorcA. 



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 spinosiim. — 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 equivalve, 

 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 inequivalve, 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 



