H6 



The Canadian Field-Naturalist 



[Vol. XXXIV. 



pcolst and gradually ascend the streams so that it 

 is even found in certain large freshwater lakes near 

 the coast and serves as food for typical freshwater- 

 fishes (lake-trout, etc.). It has not, however, in 

 the arctic becomes a freshwater species to the same 

 extent as has other Crustacea, Mysis relicta Loven 

 (see Rep. Canad., Arct. Exped., 1913-18, Vol. 

 VII, Parts B. and D.). 



Cf the three families of freshwater isopods oc- 

 curring on this continent the one (Cirolanidae) is 

 represented by a blind form in artesian wells and 

 has so far only been found in the United States, 

 and the other is that of the parasitic Bopyridae 

 found upon higher Crustacea (Decapoda). Nor has 

 this latter yet been recorded from Canada or Alas- 

 ka; the females in the genus Probop^rus become, 

 after attachment to their host, peculiarly deformed 

 and unsymmetrical while the young individuals and 

 males are more normal in habit and appearance. 

 There is a great number of marine forms of this 

 family. The third family of freshwater-isopods is 

 the Asellidae, which is represented by three genera 

 of which one has so far only been recorded from 

 the United States, and as with the species of the 

 two preceding families is apparently missing from 

 Canada and Alaska. It is found in underground 

 caves or artesian wells. The Asellidae are dis- 

 tinguished from the Cirolanidae by the fact that the 

 last pair of tail feet (uropods) are not inserted lat- 

 erally on the telson so as to form a tail-fan, but at 

 the posterior end. As mentioned above the family 

 is represented in Canada by two genera. The first 

 (Mancasellus) of these has only one species M. 

 tenax Smith, in Canada and probably does not oc- 

 cur in Alaska. It is easily separated (see Hunts- 

 man's figures) from the one species of the 

 other genus Assellus communis Say also occurring 

 in Canada by the extended, truncate epimera (seg- 

 mental processes) and by the head being much 

 broader than long and with a deep incision on each 

 side, characters which can be ascertained even in 

 very young individuals. It has thus a much greater 

 transverse diameter than has Assellus communis and 

 is superficially not unlike its terrestrial relatives 

 though lacking the latter's ability to roll itself up in- 

 to a ball. It is far less abundant than Assellus, and 

 seems to be still more retiring with mode of life, 

 being mainly found under stones, etc., nor is it found 

 in temporary pools and streams where the other 

 is often found. Its distribution in Canada seems 

 to be somewhat like that of Eucrangonyx gracilis, 

 and limited to the Great Lakes area (Superior, 

 Huron, Georgian Bay, Lake Ontario, upper St. 

 Lawrence river, the Ottawa valley and southern 

 Ontario) ; in the United States it has been recorded 



fPerhaps for breeding purpose.s ( See C.A.E. re- 

 port). 



from Lake Superior to the Detroit river. Its maxi- 

 mum length is net quite 1 '/2 cm. About its life-his- 

 tory little has been known, but I am able to give 

 some interesting data recently acquired. Thus among 

 the specimens sent me by Prof. E. M. Walker of 

 Toronto and collected near Lake Simcoe, Ont., on 

 May 30, 1917, some of the females had eggs in the 

 brood-pouch. I secured a great number of very 

 young (3-4 mm. long) individuals cf this species 

 under stones in a bight of the OUawa river on 

 July 6, 1919. 



The same day I found young Asellus communis 

 of a corresponding size, and could thus compare 

 them. The young Mancasellus could be dis- 

 tinguished from the young Asellus not only by the 

 characters given above (greater width, lateral in- 

 cision in head, etc.), but also by a very characteris- 

 tic pattern of four dark brown, longitudinal stripes 

 upon their dorsal side quite lacking on the paler 

 young of Asellus. There was some variation among 

 the different young Mancasellus in the intensity and 

 distribution of the longitudinal, dorsal stripes, but the 

 pattern was always the same, and I found it to hold 

 good also for the young (below 5 mm.) Mancasel- 

 lus which I secured at Alexandria Bay, N.Y., on 

 September 1, 1919 (see below). The full-grown 

 Mancasellus I collected, also, show some traces of 

 this pattern (the animals were dusky dorsally), so 

 it seems to be a characteristic of the young individu- 

 als. It was interesting to note, that the average size 

 of the young Mancasellus collected on July 6th 

 was larger (3-4 mm.), than that of the young Asel- 

 lus (3 mm.) on the same date; the breeds of Man- 

 casellus are probably born somewhat earlier or 

 grow faster than is the case with Asellus. From 

 observations I have made regarding the birth of the 

 young Asellus and the rather slow growth of both 

 species I can say almost with certainty, that these 

 young ones cf both species taken on July 6th are 

 about two months old. Their habits were quite the 

 same as those of the adults. 



New records of Mancasellus ienax for Canada 

 are as follows: 



Several young ones (below 5 mm.) from under 

 stones in bight canal at Alexandria Bay, Thousand 

 Islands, N.Y., Sept. 1, 1919, (F. Johansen). 



One full grown (13 mm.) male from among 

 stones; Fairy Lake, near Hull, P.Q., May 5, 1918. 

 (F. Johansen). 



One adult. Fairy Lake, P.Q., April 25, 1920, 

 (F. Johansen). 



Many specimens (various sizes) from Bridg- 

 man's Creek, Chelsea Road, Hull, P.Q., May 9, 

 1920, (F. Johansen). 



One adult (about 12 mm.) and many young 

 (3-4 mm. ones from underside of stones; bight in 



