29-i CONTRIBUTIONS TO CANADIAN" PALAEONTOLOGY. 



umbonal slopes, thence inflected rather abruptly and more or less convexly 

 inward on the ventral side and oMiquely compressed or narrowing con- 

 vexly and more gradually on the dorsal. Ventral border straight for the 

 greater part of its length : piostero-dorsal margin moderately elevated, 

 most prominent and faintly subangular a little behind the midlength : 

 hinge line short, oblique : posterior or anal margin broadly and obliquely 

 rounded, though its junction with the end the fai-thest removed from the 

 beaks is either narrowly rounded or somewhat pointed umbones promi- 

 nent, much narrower in their dorso-ventral than in their lateral diameter ; 

 beaks terminal, curved strongly inward and slightly forward. 



Surface markings and characters of the interior of the valves unknown. 

 The casts of the interior of the vahes, howevnr, are marked with a few, 

 irregularly disposed but foi' the most distant lines of growth or concen- 

 tric wrinkles. 



In the largest specimen collected (the original of tig. 5) the maximum 

 length is thirty-eight millimeti-es, the greatest height twenty-one mm., 

 and the thickness through the closed valves is estimated at twenty mm., 

 but in other specimens, such as the one represented by tigs. G and 6 a, the 

 greatest thickness considerably exceeds the maximum height. 



Pentamerus Point, Lake Manitoba, J. B. Tyrrell and J. F. Whiteaves, 

 18S8 ; ten specimens, must of which are very small. Dawson Bay, Lake 

 Winnipegosis, — on two small islands oti' Whiteaves Point (two specimens 

 from one island and one from the othei'), — on the south-west shore about 

 two miles west of 8alt Point (two specimens, one unusually large, the 

 original of tig. 5), and on the west shore at the mouth of the Red Deer 

 River (three specimens) : J. B. Tyrrell, L889. 



In this species the greatest height of the valves is invariably a little 

 behind the midlength, the dorsal margin being longer than the anal. In 

 this and in some other respects the largest specimen collected (tig. 5) difiers 

 matei'ially from the J/, ijihhosa of Hall, to which it otherwise bears a 

 certain general resemblance. Other and smaller specimens of M inflata. 

 approach nearer to some of the shorter varieties of M. citrinald, Hall, in 

 lateral outline, but the former are never as distinctly angulated on the 

 umbonal slope as the latter are said to be. According to Dr. Freeh, 

 *MytiIiircn, Hall, is exactly synunymous with Mijalimi, and, if this be 

 the case, the present species will have t(.i be called Myalitia irijiata. 



(S.) MVALINA TRIOONALIS. (N. Sp.) 



Plate 3S, fig.s. 7, 7a ami 7b. 

 Shell of medium size, subcuneiform in lateral outline, truncated scjme- 



* Zeit.schr. der Deutsch f,'eul. (iesellscli., 1S8S, vol. XL, p. 3li:i 



