149 



LAMELLIBRANCHIATA. 



CYRTODONTA HARRIETTA. (N. sp.) 



Pig. 129. 

 Fig. 129,Cyrtodonta JEfornetta. Right valve. 



Description. Transversely oblong; dorsal and ventral margins sub- 

 parallel ; posterior extremity broadly and nearly uniformly rounded ; 

 anterior extremity short, narrowly rounded, occupying only the ventral 

 half of the height of the shell, its most projecting point extending only 

 about one-twelfth the whole transverse length of the shell beyond the 

 umbones. Ventral margin slightly concave ; dorsal margin most elevated 

 at a little more than one-third the length of the shell from the posterior 

 extremity ; most projecting point of the posterior extremity a little below 

 the half height of the shell. Beaks small, spirally incurved forwards 

 down to the hinge-line nearly; shell strongly convex in the umbonial 

 region, most prominent at about one-fourth the length from the umbones ; 

 the most convex region extends from the umbones obliquely backwards 

 and downwards ; a wide shallow byssal sinus in the anterior two-thirds of 

 the length, slightly sinuates the ventral margin, and becomes obsolete at 

 one-half the height of the shell. Surface unknown. 



Transverse length 21 lines; height at the umbones 11 lines; at the 

 mosty elevated point of dorsal margin 12 lines ; depth of single valve 7 

 lines. 



Locality and Formation. English Head, Anticosti. Hudson River 

 group. 



Collector. J. Richardson. 



