of the Fishery Board for Scotland. 



229 



Between 8-5 and 10 mm. it was found that the specimens considered to 

 be dabs at that size might fit into a series leading to specimens at 10mm. 

 and onwards, which for various reasons were considered to be plaice. 



Other specimens could be found to fill the gap, but there was very 

 little difference between them all. Those which might be considered 

 plaice were, if anything, a little thinner and flatter in the body, whereas 

 the dabs were rounder ; but such difference as there was would easily 

 have been explained away as due to individual variation. 



The lines of development of the plaice and the dab Avere therefore con- 

 sidered to cross between 9 and 9*5 mm. Between 10 and 12 mm. the 

 dabs were clearly marked off from a great number of specimens which 

 were in process of metamorphosis. But these dabs did not fit into a 

 series with those of Mr. Holt, nor could his be separated from what was 

 thought to be plaice. Relying upon measurements of eye and head, 

 breadth and length, he had separated specimens at the same length and 

 the same stage of metamorphosis. His figures 109, which represents a 

 plaice, and 112, a dab, correspond so far as metamorphosis is concerned, 

 the eye being half-way. In fig. 112 the dorsal fin is not continued 

 forward to the eye, but this may have been due to accident. Such a 

 gap between dorsal fin and eye was not seen in any of the specimens in 

 the collection. 



The differences in proportions are not great. In the plaice the length 

 (without tail) is 2J times the breadth (without fins), in the dab it is 3^ 

 times ; in the plaice the length is 3|- times the length of the head, in the 

 dab it is 4 times. Mr. Holt lays stress on another proportion, namely, 

 eye to head. But this measurement is of no use, it seems to me, because 

 the eye is more subject to unequal contraction than the head, and the iris 

 (which was taken as the measure) is not round. The proportion of eye 

 to head in the plaice at these stages was found to vary from f to \. 



Another difference, the length at which pigment is assumed, is also 

 variable. In the plaice, it depends uj^on when they take to the bottom, 

 and the length at which this occurs is known to be variable. 



Mr. Holt's figures 110 and 113 correspond also, so far as the stage of 

 metamorphosis is concerned, and both seem to be plaice. On the other 

 hand, fig. 114, length 14'5mm., is in a stage corresponding with fig. 109 

 of the plaice, length 12-25mm., the eye being half-way, and seems to be 

 a dab. 



The lines of development of the plaice and the dab are certainly very 

 close, but not so close as Mr. Holt thought. With such a large number 

 of specimens the investigation at this stage became a study of variations. 

 The first point was to trace out the probable lines of development, and 

 for this there were several aids. The plaice was known to be completely 

 metamorphosed at 14mm. to 16mm., and a number of specimens at such 

 a stage were present in the collection. On the other hand, there were no 

 completely metamorphosed specimens of the dab below 17mm. From 

 the series of dabs already referred to it was evident that the dab under- 

 went metamorphosis at a greater length than the plaice. The lateral 

 line, also, is present in the plaice much sooner than in the dab. The next 

 point was to determine when metamorphosis is half complete, i.e., when 

 the pupil of the left eye is in a line with the dorsal fin. 



Sixty-two specimens were counted out at this stage, and their lengths 

 measured. There was a difficulty with some of the specimens, because the 

 fin curves as it approaches the left eye, bending round its posterior left- 

 hand margin. This occurred in the smaller specimens, the plaice. In 

 the larger specimens, the dab, the dorsal fin overhangs the left eye at 

 this stage (figs 10, 11; and fig. 114, Holt), but I am not certain how far 



