MONOMYARIA. 120 



Genus 19.— PL AGIO STOMA.— Za?«arcA'. 



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. Hg. 6. Subarcuated, the umbo of one shell 

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

 remote concentric rings. Fossil. Britain. 



Genus 20. — LIiMA. — 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. Ldiabits the British seas. 



Genus 21 — PEDUM.— Zamarc^. 



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 spondijloideiim — The Spondylus-shaped Pe- 

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

 upper valve with longitudinal stri;p, 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 



