DIMYARIA. 139 



Genus 9.— TRIGONIA.— ZamarcA. 



Generic Character. — Equivalve, inequilateral, trigonal, 

 sometimes suborbicular; teeth of the hinge oblong, laterally- 

 compressed, diverging, with transverse furrows ; two in the 

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

 valve, grooved on one side only ; ligament marginal and ex- 

 ternal. 



Trigonia pectinata. — The Toothed Trigonia. Plate 

 XVI. fig. 5. Suborbicular, with radiated or divergent, 

 prominent, and somewhat scaly ribs ; inside pearly ; margin 

 crenulated. One inch and three-fourths broad. Inhabits 

 the Australian 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 10 NUCUL A. —ZamarcA. 



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

 equivalve, inequilateral; no intermediate area; binge linear, 

 with numerous pectinated teeth, interrupted in the middle 

 by an oblique produced hollow; beaks contiguous, inflected ; 

 ligament marginal, partly internal. 



Section * Shells sublongitudinal. ** Shells transverse. 

 Nucula margaritacea. — The Pearly Nucula. Plate 

 XVI. fig. 6. Obliquely ovate, trigonal ; striae minute and 

 almost obsolete; covered with a greenish epidermis; inside 

 silvery perlaceous; margin crenulated; and having regularly 

 pectinated teeth. One half inch long. Inhabits the British 

 seas. 



Genus ll._PECTUNCULUS.— ZamarcA. 



Generic Character. — Orbicular, nearly lenticular, equi- 



