Raymond: Pelecypoda of Chazy Formation. 341 



Pamelia formation at the Hogs Back, near Ottawa, is a Ctenodonta, 

 which is much larger than C. peracuta, and wliich differs from C. 

 dubiaformis in not having the beaks centrally located. There are 

 some points in which the shell agrees with C. nasuta (Hall), but the 

 shape of the posterior end is different, and the teeth are smaller and 

 more numerous. 



Description. 



Shell oval in outline, usually flattened, but specimens from the 

 harder layers show a considerable convexity below the umbo, with 

 regular slopes to the anterior, posterior, and ventral margins. The 

 anterior and posterior margins are regularly rounded, and the pos- 

 terior end is a little narrower than the anterior. The cast shows the 

 impressions of numerous very fine teeth on the hinges, but the number 

 cannot be counted, as the beak is always flattened down upon the 

 hinge. One specimen shows five teeth on the posterior side of the 

 beak and another shows seven. The surface is marked by numerous 

 concentric lines of growth. 



Locality. — In the shale and sandstone of the Pamelia formation 

 at the Hogs Back, near Ottawa, Ontario. 



22. Cyrtodonta breviuscula Billings. (Plate XXIX, figure 17.) 



Cyrlodonla breviuscula Billings, Canadian Naturalist and Geologist, Vol. IV, 

 1859, p. 446; Whiteaves, Ottawa Naturalist, Vol. XXII, 1908, p. 107, PI. 3, 

 fig. 3- 



The type of the species is a small left valve in a bit of almost pure 

 quartz sandstone. The type has been unique until recently, when 

 the writer rediscovered the original locality "three miles east of 

 Ottawa," on a road leading toward the'river just beyond Robillard's 

 quarries. 



Description. 



Shell small, nearly as high as long. Hinge short, straight. Beak 

 near the anterior end of the shell, overhanging the hinge margin. 

 Anterior lobe small, semicircular. Basal margin straight, posterior 

 margin gently curved, oblique. Valves rather thick, the highest point 

 a little back of the umbo. A strongly elevated ridge extends from the 

 umbo to the posterior ventral angle. The type is 12.5 mm. long and 

 10 mm. high. A larger specimen is 20 mm. long and 15 mm. high. 



Locality. — The type-locality is just below the quarry in the Pamelia 



