36 COMMISSION OF CONSERVATION 



The larvae of the scallop and of the quahaug were determined by a com- 

 parative study of preserved plankton collections from Malpeque, Gaspe and 

 St. Andrews, combined with observations on the distribution of the adults 

 — the scallop with its equal measurements in length and depth corres- 

 ponding with the prodissoconch of 2 mm. scallops dredged on hydroids in 

 Gaspe bay, and the quahaug occurring in both plankton and dredgings at 

 Malpeque, but not at Gaspe or St. Andrews, and also agreeing with the 

 prodissoconch of small quahaugs 2 or 3 mm. long collected in the sand at 

 Ram Island point, Malpeque. 



Of these six species of our commonest bivalve mollusks, of which the 

 larva? were mostly determined by being traced into succeeding older 

 stages, only that of the mussel was known to me from the straight- 

 hinge period upwards. I now turned to my preserved plankton col- 

 lections and found that at Malpeque on July 11, 1904, there were 

 swarms of straight-hinge stages varying about 15 units in length. A 

 hasty and superficial examination of these, combined with their occurrence 

 in proximity to oyster beds, might easily have led to the conclusion that 

 they were all oysters. This mistake had been made by others. But they 

 are not all oysters. A few are readily identified as mussels, the rest look 

 different and may be presumed to be oysters. Upon measuring them 

 with great care it is found that they are deeper in proportion to their 

 length than the mussels, but that, besides, some of them appear rounder 

 than others, due to their having a shorter hinge-line. There are in reality 

 two different species of deep ones. Gaspe and St. Andrews plankton was 

 re-examined, and it was found that those with the shortest hinge-line do 

 not occur at these places, but that the other deep ones do. At once it 

 was suggested that the former are oysters and the latter clams, corres- 

 ponding with the distribution of adults. Selecting examples of exactly 

 the same length the dimensions are: 



Length Height Hinge 



Mussel 15 10 11 



Clam 15 13 10 



Oyster 15 13 7.5 



The eye can easily perceive a difference in the proportions of a mussel 

 as compared with a clam, but it requires a certain refinement of judgment 

 to do the same for a clam and an oyster. Faithful selection, examination 

 and measurements have filled up the intervals between these small straight- 

 hinge larvse and the large umbo-stages sufficiently to satisfy me that these 

 ol^irvations are correct. Moreover, I have since pursued the subject 

 backwards to smaller plankton stages and find it holds good. 



Adults are easily distinguished; the full-grown larva? less easily, for, 

 since they bear little resemblance to the corresponding adults, other marks 

 of distinction have to be selected; but the young larvse are still more diffi- 



