No. 505] 



THE CANADIAN OYSTER 



11 



For each of the commonest species a table of lengths was 

 prepared, jumping only one of the smallest units of my 

 ocular micrometer at a time, and the heights were filled 

 in as individuals of these lengths occurred. Thus larvae 

 of the mussel, the clam and the oyster, at the period we 

 are considering, measure as follows: 



—a table which will immediately make apparent the 

 truth of many of my statements. The eye can easily per- 

 ceive a difference in the proportions of the mussel and 

 the clam, but it requires a certain refinement of judg- 

 ment to do the same for a clam and an oyster. Such an 

 oyster larva actually measures .103 x .089 mm. in length 

 and height, and has a short, slightly concave hinge-line 

 of scarcely half the length of the shell. 



I have said that in collections of straight-hinge larvae 

 but one in a great many is an oyster. A similar state- 

 ment might be made for any period in the larval existence 

 of the oyster. Upon one occasion when the umbo-stage 

 was most abundant I estimated that there was only one 

 oyster among twenty-five bivalve larvae. Another time 

 I found that when the plankton net was towed at the sur- 

 face against a wind it caught about a quarter as many 

 oysters as in going back over the same distance with the 

 net sunk a few feet below the surface of the water. 



I am of opinion that the study of plankton collections 

 for bivalve larvae will be found a most useful help in de- 

 termining the breeding season — that is to say the height 

 of the breeding season. From the foregoing pages it 

 may be concluded that oyster larvae are present in the 

 water from the eleventh of July to the first September, 

 and that oyster spat are present from the sixteenth of 

 August. This would seem to indicate that the second 

 half of August is taken up with the last stages of growth 

 of late larvae and that the period of growth of the masses 



