180 CONTRIBUTIONS TO CANADIAN PALEONTOLOGY. 



centric fiirrowB. In addition to these, in each valve an elevated but 

 narrow linear ridge runs obliquely backward from the jiosterior side of 

 the beaks to a little behind the centre of the ventral margin. 



Posterior muscular impression narrowly siibelliptical, placed very 

 high up, almost withiu the escutcheon, and a little behind the mid- 

 length ; anterioi' muscular impression, pallial line and accessory valves 

 unknown. The pedal opening in front seems to have been largo and 

 broadly rhomboidal in outline. 



The measui-ements of the only specimen collected are approximate- 

 ly as follows : maximum length, about fifty-one millimetres ; greatest 

 height, as measured in the centre, immediately behind the umbones, 

 and maximum thickness through the closed valves, both thirty-one mm. 



North Saskatchewan Eiver, Township 54, Eange 2, west of the 4th 

 Principal Meridian, 188fi : one nearly perfect and well preserved cast 

 of the interior of the closed valves, with a small portion of the test 

 adhering to the left valve. An apjiarently well characterized and very 

 distinct species. 



Since the above description was written, another specimen of this 

 species, the exact locality of which is unfortunately unknown, has 

 been presented to the Museum of the Survey by the Historical and 

 Scientific Society of Manitoba. 



In this specimen, which is represented on Plate 25, fig. 2, and 

 which retains a considerable portion of the test, especially on the right 

 valve, the concentric markings would p)erhaps be better described as 

 raised lines of growth, which are very faint and almost obsolete pos- 

 tei'iorly but prominent and well defined anteriorly, rather than as 

 " ridgedike folds." On the tumid anterior portion of each valve these 

 concentric lines of growth are crossed by faint radiating ribs which 

 give to the former a somewhat tuberoulatcd appearance. In this 

 specimen, too, the slightly elevated median ridge which runs obliquely 

 backward from the beak, is longitudinally and very narrowly grooved, 

 at any rate on the light valve. 



GASTEROPODA. 



Hydatina parvula, Whiteaves. 



Plate 24, figs. 5 and 5a, 

 Hydalina parvula, Whiteaves. 1887. Op. cit. ,p. 158 E. 



Shell small, the outer whorl enveloping all the preceding volutions, 

 strongly inflated and very ventricose, so much so that its maximum 

 breadth is very little less than the entire height or length, — subtruncated 

 posteriorly, broadest above or behind the middle, narrowing rapidly 



