53 



T. Terminalis has the lower valve more elevated and a deeper notch 

 for a foramen ; T. Huronensis has the lower valve concave. 

 Locality and Formation. Montreal. Trenton limestone. 

 Collector. Sir W. E. Logan. 



TREMATIS OTTAWAENSIS. 



Fig. 58. 



Description. Nearly circular ; length a little greater than the width ; 

 upper valve moderately and uniformly convex, most elevated about the 

 middle ; apex small, obtusely pointed, slightly elevated, marginal. Surface 

 with fine, radiating striae, which increase by interstitial addition, sometimes 

 closely crowded together, in which case there are ten or twelve in the 

 width of one line ; occasionally more distant, or from four to eight in one 

 line. The intermediate grooves are divided into square compartments by 

 cross ridges, which connect the radiating ridges, but are not continuous, 

 those in one groove not corresponding in position with those in the adjacent 

 grooves, so as to form uninterrupted concentric lines. In specimens with 

 the striae closely crowded together, only the radiating lines are distinctly 

 visible, but the others can always be detected in good^specimens, on close 

 examination. 



Length, from 12 to 15 lines ; width a little less than the length. Lower 

 valve unknown. 



The species figured by Prof. Hall, under the name of Orbicula filosa 

 resembles this, but has the surface much more finely striated and not reti- 

 culated. This is also a larger species. 



A large number of specimens of this species were found in one spot 4 

 or 5 yards in extent, in a thin stratum of argillaceous shale between beds 

 of Trenton limestone at Ottawa. None of them were perfect, and they 

 appeared to be all of the same value. 



Locality and Formation. City of Ottawa. Trenton limestone. 



Collector. E. Billings. 



TREMATIS HURONENSIS. (N. sp.) 



Fig. 59. 



Description. Obtusely oval. Lower valve gently concave ; foramen a 

 deep triangular notch in the posterior margin, extending inwards a little 

 more than half the distance to the centre. Upper valve, moderately con- 

 vex, most elevated in the posterior half, with a small rounded beak, in- 

 curved down to the plane of the posterior margin. Surface with fine very 



