FRESH-WATER MALACOSTRACA OF ONTARIO 149 



SESSSONAL PAPER No. 39b 



commoner variety and M. tenax dilata (Smith) 1874, p. 661, The latter differs 

 from the former in being broader, in having more joints in the flagellum of the 

 antennule, and in having three teeth (the middle one largest) on the palmar margin 

 of the propodus of the first gnathopod of the adult male (see Fig. 2). I am unable 

 to definitely separate these, either as to the width of the body, number of joints 

 in flagellum of the antennule or in the shape of the first gnathopoda of the male. 

 The extremes in the conditions of the gnathopoda are shown in Fig. 2. In some 

 lots of specimens, one or other form appears to predominate and in others all 

 gradations between the two extremes are to be seen. The typical dilata I have 

 seen only from the north end of the Georgian Bay (Fitzwilliam Id.). Smith's 

 specimens came from the Detroit River. 



Order AMPHIPODA. 



Miss Weckel (see bibliography) has recently given an account of the fresh- 

 water species of this group occurring in North America. Six species have been 

 reported from the region of the Great Lakes, although only three have actually 

 been recorded from Canada. The Amphipods occur at practically all depths, 

 either crawling about among debris or swimming freely near the bottom. Only 

 rarely do they venture out into the open water. 



Key to the Genera. 



Ai Last thoracic leg shorter than preceding one and with basal joint large and 



leaf like. (Fig. 3) Pontoporeia. 



Aj Last thoracic leg longer than preceding one and with basal joint little larger 

 than that of preceding one. 

 Bj Telson cleft to base. Third uropod biramous, rami nearly equal (Fig. 4, b). 



Gammarus. 



Bj Telson notched. Third uropod biramous but inner ramus rudimentary 



(Fig. 4, c) Eucrangonyx. 



Bg Telson entire. Third uropod uniramous (Fig. 4, d) Hyalella. 



Pontoporeia hoyi (Stimpson Mss.) Fig. 3. 



P. affinis Smith, 1871, p. 452. 

 " " Nicholson, 1872, p. 501. 

 " hoyi Smith, 1874, p. 647. 

 " " Weckel, 1907, p. 26. 



This species occurs in abundance on muddy or gravelly bottoms at various 

 depths down to 169 fathoms (Smith). In Lake Superior, according to Smith, 

 it is found in as shallow water as 4 fathoms. It is the same at the north end of 

 Georgian Bay, where it was dredged last summer (1912) by Messrs. Robertson 

 and Wodehouse in Rattlesnake Harbour, Fitzwilliam Id. In this harbour many 

 whitefish are caught in pound nets, and they doubtless feed upon this species in 

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