]22 DIMYAIRA. 



Genus 9 — TRIGONIA.— Lawa?-^. 



Generic Character. — Equivalve, inequilateral, trigonal, and 

 sometimes orbicular; teeth of the hinge oblong, laterally com- 

 pressed, diverging, with transverse furrows ; two in the right 

 valve, grooved on both sides ; and four in the left valve, grooved 

 on one side ; ligament marginal and external. 



Trigonia pectinata. — The Toothed Thigonia. Plate 

 XIV. fig. 5. Suborbicular, with radiated or divergent, promi- 

 nent, and somewhat scaly ribs ; inside pearly ; margin crenu- 

 lated. One inch and three-fourths broad. Inhabits the Aus- 

 tralian seas. 



TRIBE III. ARCACEA. 



Primary teeth small, numerous, and disposed in a line in 

 each valve, which is either straight or interrupted, the teeth of 

 one valve fitting into the intermediate spaces of the other valve. 



Genus IQ.—^UCVhk.— Lamarck. 



Generic Character. — Transverse, oval-trigonal, or oblong, 

 equivalve, and equilateral •, no intermediate area ; teeth of the 

 hinge linear, numerous, and pectinated, interrupted in the middle 

 by an oblique produced hollow ; beaks contiguous, reflected ; 

 ligament marginal and partly internal. 



Section * shells sublongitudinal. ** Shells transverse. 



Nucula margaritacea. — The Pearly Nucula. Plate XVI. 

 fig. 6. Obliquely ovate, trigonal ; striae minute and almost ob- 

 solete ; covered with a greenish epidermis ; inside silvery pear- 

 laceous ; margin crenulated ; and having regularly pectinated 

 teeth. One half inch long. Inhabits the British seas. 



Genus 11.— PECTUNCULUS.— ZawarcA. 



Generic Character. — Orbicular, doubly convex, equivalve, 

 subequilateral, and close ; hinge arcuated, teeth numerous, 

 oblique, serrated, alternately inserted into the opposite valves, 

 middle ones obsolete ; ligament external. 



