150 



Ctrtodonta Emma. (N. sp.) 



Fig. 130. 

 Fig. 130. — Cyrtodonta Emma. — Right valve. 



Description. — Transversely elongate, ovate, slightly narrowed from the 

 umbones backwards. Anterior extremity occupying the lower two-thirds 

 of the height, rounded, and projecting a little more than one-sixth of the 

 whole transverse length in advance of the umbones ; posterior extremity 

 rounded, with indications of an oblique truncation La the upper half. 

 Dorsal and ventral margins sub-parallel, but slightly converging backwards. 

 Beaks, as is apparent by the cast of the interior, closely incurved, umbones 

 rather strongly convex, slightly flattened by a shallow byssal sinus, which 

 extends to the ventral margin and renders the same gently concave. The 

 greatest convexity of the shell is at one-third the length, or a little more, 

 from the anterior extremity. The dorsal margin is slightly compressed in 

 its posterior half. 



Surface of shell unknown, but probably concentrically striated. The 

 casts of the interior shew some strong concave or sub-angular concentric 

 wrinkles on the umbones, and curving round in front of them. 



Transverse length 18 lines ; height at the umbones 9 lines ; at the 

 posterior extremity of the hinge-line, 7 lines ; depth of a single valve 5 

 lines. 



Locality and Formation. — English Head, Anticosti. Hudson River 

 group. 



Collector. — J. Richardson. 



Ctetodonta ponderosa. (N. sp.) 



Description. — Oblong, oval, somewhat sub-rhomboidal, dorsal and ventral 

 margins sub-parallel; the hinge-line straight with a weU-deflned area, 

 sometimes two lines in width ; the posterior half of the margin forms a 

 rounded obtuse angle (130° to 145°) with the hinge-line, then straight 

 but declining a httle downwards for a length a little greater than that of 

 the hinge-line when it forms another rounded angle of about 150° (the 



