DIMYARIA. 139 



Genus 9.— TRIGONIA.— Zamarc^. 



Generic Cliaracter 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 NUCULA. — Lamarck. 



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

 equivalve, inequilateral; no intermediate area; hinge 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 ti'ansverse. 



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 



Genus II.— PECTUNCULUS.— Za;«a;-c-^. 



Generic Character. — Orbicular, nearly lenticular, equi- 



