328 ICHTHYOLOGTCAL CONTRIBUTIONS. 



through the tissues of the base of the anterior part of the dorsal 

 fin. It is well known that Steenstrup in 1863 described transition 

 stages of Pleuronectids^ obtained from the North Atlantic, in which 

 the eyes, after metamorphosis, were on the left side, and the right 

 eye passed through the head to reach that side. He considered 

 these stages to belong to the genus Plagusia. Agassiz in 1878 

 (Proc. Amer. Acad. Arts and Sci., vol. xiv) described transition 

 stages, quite similar to those of Steenstrup, captured at the mouth 

 of Newport Harbour, and ascribed them likewise to the genus 

 Plagusia. Emery, the Italian ichthyologist, has pointed out that 

 these specimens of Steenstrup and Agassiz certainly do not belong 

 to the genus Plagusia, because in the latter the dorsal and post- 

 anal fins are continuous with the caudal, and in these specimens 

 they are quite distinct and separate. Without discussing the ques- 

 tion at length, or carefully examining the evidence, Emery suggests 

 that the North Atlantic specimens belong to the genus Rhomboid- 

 iehthys. Emery, in the same paper, describes another larval form in 

 which the longitudinal fins are continuous with the caudal, and the 

 right eye passes through the base of the dorsal fin to the left side 

 of the head. He did not succeed in identifying this larva with any 

 known adult species. The larvae in which this process of perfora- 

 tion has been hitherto described are sinistral, the eyes are on the 

 left side ; but nevertheless, considering the great anterior prolonga- 

 tion of the dorsal fin in the adult sole, it seemed not impossible that 

 the migration of the lower eye should take place in that species also 

 by perforation. The larva now described proves that this is not the 

 case, its dorsal fin being still behind the left eye. The left eye has 

 not quite reached the edge of the head ; it is still on the left side, 

 but it is very near the edge ; and when the larva is examined on a 

 slide, lying flat on its left side, the cornea of the left eye is seen to 

 project slightly beyond the edge of the head, as seen in the figure. 



The next important feature in this larva is the presence of an 

 air-bladder of considerable size. Hitherto, so far as I am aware, 

 an air-bladder in larval Pleuronectidae has only been observed in the 

 turbot and brill. I have never seen a trace of it in species of 

 Pleuronectes. In that stage of the flounder which corresponds to 

 the stage of 8olea here described, and which I have frequently 

 examined, no trace of an air-bladder is visible (compare the pi. 

 xvii, fig. 5, of my Treatise on the Sole). No air-bladder is present 

 in the adult sole ; and in the stage just after the completion of the 

 metamorphosis, when the little sole is 12 to 15 mm. long, the organ 

 has already disappeared (see pi. xvi, fig. 5, op. cit.). 



It is interesting to notice that in this larval stage some of the 

 specific characters are already developed. I refer especially to the 



