256 



" Surface marked with a few impressed and concentric strise of growth ; 

 test rather thick. 



" Hinge with both cardinal and lateral teeth ; muscular impressions 

 unknown. 



" Portage road at falls : one testiferous left valve. 



" Mr.TJlrich thinks that this shell is " closely related to, if not quite the 

 same as, Amhonychia aphma, Hall," from the Niagara limestone of Illinois, 

 which he (Mr. Ulrich) referred to Mytilarca in 1894, in the seventh 

 volume of the Reports of the Geological Survey of Ohio. A. aphoea, how- 

 ever, was based upon a mere cast, which does not show the proportionate 

 length of the hinge line, the size of the cardinal area, nor the surface 

 markings, so that it is scarcely possible to make a satisfactory comparison 

 between it and the specimen from the Ekwan River.'' 



Ctenodonta subovata, Whiteaves. 



Plate 27, figs. 9 and 9 a. 



Ctenodonta subovata, Whiteaves 1904. Geol. Surv. Canada, Ann. Rep., vol. 



XIV, pt. F, p. 47. 



" Shell small, inequilateral, moderately convex, subovate and one fourth 

 longer than high. Anterior (?) side short and rounded ; posterior (?) side 

 produced, a little longer, and more narrowly rounded at its outer ter- 

 mination ; ventral margin gently convex ; superior border sloping abruptly 

 downward in front of the beaks and much more gradually so behind them ; 

 umbones small and moderately prominent : beaks also small, incurved 

 and placed in advance of the midlength ; ligament external, short, placed 

 on the shorter end of the hinge line. 



" Surface faintly, very minutely and concentricallj' striated. 



" Hinge dentition and muscular impressions unknown. 



" Dimensions of the only specimen collected : maximum length, twenty 

 millimetres ; greatest height, fifteen mm. and a quarter ; maximum thick- 

 ness, ten mm. and a quarter. 



" Portage road at falls : one testiferous specimen, with both valves. 



" The homologies of the shell of Ctenodonta are unknown, and it is not 

 at all clear which is the anterior and which the posterior side of this 

 species. If the shorter is the posterior side, as in Nucula and as would 

 seem to be indicated by the position of the ligament, then the beaks of 

 this species are placed a little behind the midlength and vice versa. 



"In outline this shell agrees very nearly with my C. simulatrix and 

 less closely with C. Alhertina, but these species had the ligament on the 

 longer, instead of the shorter end of the hinge." Ulrich. 



