DIMYARIA. 151 



Mysia rotundata.— The Rounded Mysia. Plate XVII. 

 fig. 6. Orbicular, somewhat convex, thin, sub -pellucid, 

 and obscurely striated ; umbones small and oblique. Three 

 quarters of an inch long. Inhabits the British seas. 



Genus 35 — CORBIS.— Cuvier. 



Generic Character. — Transverse, equi valve, without 

 flexuosity at the anterior margin, slightly depressed ; beaks 

 small and incurved ; hinge with two primary teeth, and two 

 lateral, of which the posterior is nearest the cardinal teeth ; 

 muscular impressions simple ; ligament external. 



Corbisfimhriata. — The Fringed Corbis. Plate XVII. 

 fig. 8. Very thick, white, transversely oval, gibbous, longi- 

 tudinally striated, with transverse undulated furrows ; the 

 margins crenulated ; with depressions somewhat lanceolate. 

 Two inches and a half broad. Inhabits the Indian ocean. 



Genus 36.— ARCOPAGIA.—ZeacA. 



Generic Character. — Transverse, equivalve ; right valve 

 sub-depressed; umbones very small, and nearly straight; 

 with two primary teeth in each valve, each of the larger 

 ones being cleft; left valve with two remote lateral teeth, 

 that on the anterior slope large, with a sinus betwixt it and 

 the margin for the reception of the lateral tooth in the oppo- 

 site valve ; muscular impression extremely large, and defined 

 by a deep irregular groove ; near its lower centre are several 

 strong punctiform marks ; margin very broad, well defined, 

 and glossy ; ligament subexternal. 



Arcopagia crassa. — The Thick Arcopagia. Plate 

 XVII. fig. 9. Suborbicular, strong, thick ; upper valve 

 flat ; with coarse, thick-set, transverse striae ; pale yellow, 

 radiated longitudinally ; inside white, with a large patch of 

 yellow or pink; muscular impressions very deep. One inch 

 and a half long. Inhabits the British seas. 



