240 CONTRIBUTIONS TO CANADIAN PALAEONTOLOGY. 



Dimensions of the onlj^ specimen collected, maximum length, estim- 

 ated at twenty seven millimetres ; greatest height, twenty five mm. ; 

 maximum thickness through the closed valves, a little over sixteen 

 mm. (16.3 mm.) 



Athabasca River, first ten miles below the mouth of the Clearwater, 

 Dr. E. Bell, 1882 : one nearly perfect and well preserved specimen, 

 with both valves. 



'I'his species would seem to be a very aberrant member of the genus 

 Ptychopteria, for its shell is equivalve and its posterior wing is devoid 

 of any longitudinal fold or groove and merges imperceptibly into the 

 unibonal convexity. 



It appears to be most nearlj^ related to the F. expansa of Hall,* from 

 the Chemung Group of Pennsylvania, but its beaks are more nearly 

 central, its posterior wing is not nearly so much produced above, and 

 the surface markings of the two species are quite different. 



Leptodesma Demus, Hall. (Var.) 



Plate 32, fig. 7. 



Leptodesma Vemus, Hall. 1884. Pal. St. N. York, vol. V, pt. 1, Lamellibr., 1, 

 p. 203, pi. xc, figs. 15 and lli. 



Athabasca River, fii-st ten miles below the mouth of the Clearwater, 

 Dr. R. Bell, 1882 a pei'fect left valve which is almost exactly similar 

 in shajje to the right valve of L. Denus figured by Hall (op. cit., pi. 

 xe, fig. 15), except that its basal margin is not quite so narrowly 

 rounded. 



The types of i. Demus are stated to be from the Chemung Group of 

 Lawrenceville, Tioga Co., Pa. 



Leptode^.-ma Jason, Hall. 



Plate 32, fig. 8. 



LeplodeKiiia Jaaun, Hall. 1884. Pal. St. N. York, vol. V, pt. 1, Lamellibr., 1, p. 

 213, pi. xci, figs. 4-(i. 



Athabasca River, opposite La Saline, R. G. McConnell, 1890 : a few 

 good specimens, one of which has both of the valves preserved. 



The types of i. Jason are also .stated to be from the Chemung Group 

 of Pennsylvania. 



• Pal. State N. York, vol. V, pt. 1, Lamellibr., 1, (1881), i.. 152, pi. xxiii, ligs. 10 and II. 



