CONCHIFERA. 
455 
Mysia rotundata. — The Rounded Mysia. 
Plate XC. fig. 6. 
Orbicular, somewhat convex, thin, sub-pellucid, and obscure¬ 
ly striated ; umbones small and oblique. Three quarters of an 
inch long. Inhabits the British seas. 
Genus 35. — C ORB IS.— Cuvier. 
Generic Character —Transverse, equivalve, slightly depress¬ 
ed ; beaks small and incurved ; hinge with two primary teeth, 
and two lateral ones, of which the posterior one is nearest the 
cardinal teeth; muscular impressions simple; ligament exterior. 
Corbis fimbriala. — The Fringed Corbis. 
Plate XC. fig. 8. 
Very thick, white, transversely oval, gibbous, longitudinally 
striated, with transverse undulated furrows; the margins crenu- 
lated ; with depressions somewhat lanceolate. Two inches and 
a half broad. Inhabits the Indian ocean. 
Genus 36.—ARCOPAGIA.— Leach . 
Generic Character. —Transverse, equivalve; right valve sub¬ 
depressed ; umbones very small, and nearly straight; with two 
primary teeth in each valve, both 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 recep¬ 
tion of the lateral tooth in the opposite valve; muscular impres¬ 
sion extremely large, and defined by a deep irregular groove ; 
near its lower centre are several strong punctiform marks; mar¬ 
gin very broad, well defined and glossy; ligament subexternal. 
Arcopagia crassa .— The Thick Arcopagia. 
Plate XC. 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. 
Genus 37 TELLINIDES.— Lamarck. 
Generic Character _Transverse, inequilateral, flattened, a lit- 
