MONOMYAIRA. 113 



Genus 19,— PL A GIOSTOMA.— Za/warc/t. 



Generic Character. — Subequivalve, free, somewhat eared; 

 base of the hinge transverse and straight ; beaks remote ; hinge 

 without teeth ; depression, for the reception 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. — Equivalve, with small ears ; valves gap- 

 ing 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 ; lovver margin denticulated. One 

 half inch long. Inhabits the British seas. 



Genus 2\. — VE'D\JM.— Lamarck. 



Generic Character — Inequivalve, subeared ; the lovver valve 

 tumid ; beaks unequal, separate ; hinge without teeth ; ligament 

 partly external, inserted in an elongated groove, in the internal 

 surface of the beaks ; lovver valve, with a notch near the poste- 

 rior margin. 



Pedum spondyloideum. — The Spondylus- Shaped Pedum. 

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

 valve with longitudinal striae, white, granulated and rough, slight- 

 ly tinged with purple near the beak. Two inches and a half 

 long. Inhabits the Indian ocean. 



Section III. — Ligament marginal and eloRgated. 



TRIBE I. JNIALLEACEA. 



With foliaceous shells, more or less inequivalve ; having the 

 K 3 



