tJNIO SULCATUS. 



SPECIFIC CHARACTER. 



Transversely subquadrate ; posterior margin not extend- 

 ed beyond the umbones. 



SYNONYM. 



U. SULCATUS. Lea. Trans. Jimer. Philos. Soc, vol. 

 iii. N. S. 



DESCRIPTION. 



Shell very obliquely transverse subquadrate ; with an 

 indented groove extending from the umbo to the anterior 

 and anterior basal margins, between which the edge is 

 somewhat retuse ; epidermis yellowish-olivaceous, with ra- 

 diating, deep green, more or less undulated lines ; summit 

 near the posterior extremity, prominent : anterior margiii 

 with two obtusely rounded angles, a little gaping : cavity 

 of the hinge membranes behind the summits, deeply exca- 

 vated, nearly as long as wide, distinctly angulated behind : 

 posterior margin regularly rounded, remarkably short, 

 hardly extending beyond the line of the posterior tip of 

 the umbo : ivithin perlaceous-white, more particularly iri- 

 descent before : teeth regular, anterior cardinal tooth paral- 

 lel with the lamelliform tooth, which is very slight arqua- 

 ted : posterior muscular impression deep : sinus of the car- 

 tilage very slight, regular : cavity of the umbo not deep, 



PLATE v. 



