255 



in regard to them, thinks that Leptodomus undulatus is an Ambonychia 

 allied to A. planistriata, Hall, and that the former had fine surface radii. 



Ambonychia septentrionalis, Whiteaves. 

 Plate 28, fig. 5. 



Ambonychia scptentrionaHs, Whiteaves 1904. Geol. Surv. Canada, Ann. Rep., vol. 



XIV, pt. F, p. 46. 



" Shell obliquely and acuminately subovate or subrhomboidal, very 

 inequilateral, rather strongly convex, most prominent in the umbonal 

 region of each valve. Anterior side very short, abruptly truncated, or 

 rather inflected, and flattened ; posterior side a little longer, broadly 

 rounded at its extremity and forming a subangular junction with the 

 hinge line above. TJmbones prominent, tumid but rather narrow ; beaks 

 incurved, anterior, and almost if not quite terminal ; hinge line straight 

 behind the beaks, equal to about two thirds of the greatest length of the 

 valves beneath. 



" Surface marked with a few faint and obscure concentric undulations 

 and lines of growth, also by extremely minute radiating lines. Test very 

 thin. 



" Hinge dentition and muscular impressions unknown. 



" Portage road at falls : a cast of the interior of both valves, with part 

 of the test preserved. 



" This shell is rather similar to the A. affinis of Ulrich from the Middle 

 Galena of Minnesota and Illinois, both in its shape and surface markings. 

 But, in the former the posterior end is more broadly rounded and not so 

 much produced below, and the radiating raised lines of the surface are 

 much more minute." 



Mytilaeca peenoides, Whiteaves. 

 Plate 27, fig. 8. 



Mytilarca pernoides, Whiteaves 1904. Geol. Surv. Canada, Ann. Rep. , vol. 



XIV, pt. F, p. 47. 



" Shell compressed convex, rather obliquely subovate and very inequi- 

 lateral, or broadly mytiloid and subalate behind. Anterior side very 

 short, truncated or abruptly inflected above and rounded below ; posterior 

 side a little longer, its outer margin truncated somewhat obliquely and 

 forming an angular or subangular junction with the cardinal border above, 

 but rounded below. Cardinal border behind the beaks straight, its entire 

 length equal to fully two thirds or more of the greatest length of the 

 valves below ; hinge area large ; umbones apparently not very prominent ; 

 beaks appressed, incurved and almost terminal. 



